1 Introduction

Growth and Characterization of 2.xµm VECSELs on GaSb L.Boumaa, S.P.R.Clarkb, P.Ahirwarb, C.Hainsb, G.Balakrishnanb a Department of Physics & Astronomy, University of Southern California, Los Angeles, CA 90089 b Center for High Technology Materials, University of New Mexico, 1313 Goddard SE, Albuquerque, NM 87106 REU Paper, submitted August 3rd, 2012

Abstract The goal of this project was to grow and characterize 2.xµm VECSELs within the III-Sb material system. We grew QW-based active regions on GaSb substrates, using AlSb cladding layers, Al0.25Ga0.75Sb barrier layers and 9 In0.2Ga0.8Sb QWs, along with about 5nm of GaSb on top to prevent oxidation in the AlSb cladding layer. For the DBR, we grew 19 pairs of AlAs0.08Sb0.92/GaSb DBR on the GaSb substrate. Once the sample chips were characterized (SEM, XRD, and FTIR reﬂectivity for the DBR, XRD and PL for the active region), we assembled a lasing setup to attempt to optically pump the chip with a 75W diode laser. We stuck the chip to a Cu-Ag-Al heat sink with Mung paste (a silver grease), and attempted to achieve lasing. Ultimately, we ran short on time for our project, and were unable to achieve lasing action. However, previous research from Balakrishnan et al. was used to exemplify expected results.

1 Introduction

Upon its realization in 1960, the first working laser was labeled as a “solution looking for a problem” [1]. But the convenience of having control over a narrow, monochromatic (near-single wavelength), coherent beam of light soon became evident to researchers, the government, and the public alike. Half a century later, lasers are now ubiquitous; they are omnipresent in consumer electronics, information technology, scientific research, medicine, law enforcement, the military, and even cosmetic enhancements like hair removal and acne treatment. Moreover, classes of lasers beyond T. H. Maiman’s original design have been developed. Maiman’s laser was only capable of pumping his active material (ruby - chromium in corundum) for a few milliseconds, so although he achieved operation at 694.3nm, it was only in short flashes [2]. Continuous wave (CW) operation is more sought after today, but Maiman’s technique of rapidly pumping a laser developed into mode locking and q-switching, techniques that can deliver incredibly high levels of power on femto (10−15) and even attosecond (10−18) scales. Further, lasing is obtainable outside of Maiman’s solid-state laser design. Today, we have gas lasers, diode lasers, photonic crystal lasers, and the specific topic of this report: 2.0µm VECSELs grown on GaSb substrates. We cultivate different classes of lasers for different uses. One major factor in laser design is desired output wavelength and tunability - that is, what part in the electromagnetic spectrum you want to achieve lasing at. Commercially available lasers are mostly available between the ultraviolet (UV, λ ≈ 200nm) and far-infrared (FIR, λ ≈ 700µm), but xasers (x-ray emitting lasers) are also available in laboratory settings (see Figure 21 in the appendix). In the mid-wavelength infrared (MWIR) range (λ ≈ 2 − 5µm) that we will be working in, high-power, high-quality lasers have applications in remote sensing technologies (eg: LIDAR), gas absorption spectroscopy (useful for climate monitoring), and free-space optical communication; applications thus exist both in science and also in the military.

1 Since their proposal in the early 1990s [3], vertical external-cavity surface-emitting lasers (VECSELs), also known as optically-pumped semiconductor disk lasers (OPSDL), or just semiconductor disk lasers (SDLs) have been significantly developed by the scientific community for these very uses. Unlike other laser systems that emit in short and mid-infrared wavelengths, optically pumped VECSELs can be designed to scale to high powers while maintaining low beam divergence, all while having broad wavelength tunability. Further, due to the external nature of the VECSEL’s cavity (in contrast to say, the VCSEL system that preceded the VECSEL), VEC- SELs are capable of incorporating intra-cavity elements for various ends; take for example using nonlinear crystals to attain frequency doubling and quadrupling. It’s no surprise that they have been called the “ultimate disk-laser” [4] - they combine all this functionality while being compact and relatively efficient. In short, VECSELs provide:

High beam quality (nearly TEM00), in both CW and short-pulse generation Broad tunability - you can choose your semiconductor material systems to lase all throughout the visible [5] [6] and IR [7] spectra, and when using quantum dots (QD) or quantum dashes (QDash) can tune the emission of a single SDL structure to a greater degree than with QWs Easy power scalability - since you are optically pumping your chip, increasing your pump- beam diameter, and thus the gain area of your active region, will increase lasing power. Reaching tens of watts while operating CW is very feasible

The ability to incorporate intracavity elements (usually nonlinear crystals for frequency doubling/quadrupling, but also birefringent ﬁlters for tuning) Potential for compactness

While major challenges in realizing the potential of VECSELs include minimizing the quantum defect, meeting growth constraints (lattice matching layers, QW placement, getting a good gain- microcavity alignment), and thermal management. For the latter, using eﬀective heat dissipation systems is critical to prevent heat spreading from limiting the maximum output power of the VECSEL.

2 Background

2.1 What is a VECSEL?

Firstly, a laser is an optoelectronic device that emits light through an amplification process based on the stimulated emission of photons. “Laser” itself is an acronym, standing for (L)ight (A)mplification by (S)timulated (E)mission of (R)adiation. Lasers are useful because the light they emit is monochromatic, has a high degree of directionality, and it is coherent (the waves of light are consistently in phase with each other). The basic operating principles of a laser (stimulated emission, population inversion, optical resonators) are presumed background knowledge. VECSELs are vertical external-cavity surface-emitting lasers. They are a subset of semiconductor lasers that have their resonator cavity formed between a high-reflectivity distributed Bragg

2 reflector (DBR) dielectric mirror and an external spherical mirror, also known as an external output coupler (OC). The gain for the laser is provided by a quantum well (QW) active region which is grown on top of the DBR (there has also been experimentation with quantum dot and quantum dash active region designs, but they are beyond the scope of this report). The QWs in the active region are evenly spaced in what is known as a resonant periodic gain (RPG) arrangement. VECSELs operate as shown in Figure 1: use an external laser (in our case, we use a diode laser) to optically pump a semiconductor chip. The semiconductor chip contains a QW-based active region on top of the DBR, so the high-energy incident pump photons are absorbed in separate pump-absorbing layers that double as quantum well barriers. Back-reflection from the on-chip laser cavity mirror (ie: the DBR) sets up an intracavity electrical field standing wave (see Figure 13 - discussed in more detail later) between the DBR and the semiconductor-air interface due to the high refractive index difference between air and the semiconductor, which causes partial reflection. The pump photons create electron-hole pairs in the absorbing region, which are confined in 2 spatial dimensions to the smaller bandgap QWs and then recombine in the QW region to emit lasing photons with lower energy than the incident photons (the difference between the two energies is known as the quantum defect). Note that the output coupler which completes the cavity must also have a fairly high reflectivity (usually coupling between 1% and 4%, compared to the DBRs 99.99% reflectivity at emission wavelength), since the gain from a single pass through the QWs is at most a few percent.

Figure 1: Schematic of a simple VECSEL. The angled pump beam creates an elliptically excited active region. The cut-away inset shows the active region, DBR, and heat-spreader. Source: [4]

2.1.1 Surface v. Edge Emitters

As a note on the emission geometry of the VECSEL, it is important to understand that VECSELs are surface-emitting instead of edge-emitting (Figure 2 illustrates the diﬀerence). Edge emitting lasers use a waveguide to conﬁne light to the plane of the semiconductor chip and then emit light from the edge of the chip. The output beam from these chips (if it is going to remain in single-transverse mode operation) has a wider dimension in the plane of the chip, usually around

3 1 by several microns. Although it is possible to reach up to several hundred mW of output power, the beam’s wider dimension in the chip’s plane often results in asymmetric and angular divergence of the beam [8]. Conversely, surface-emitting lasers emit light orthogonal to the plane of the laser chip. The beneﬁt of this lies in the fact that their beam has a circular cross section and larger beam size, and thus the beam is both symmetric and also has less divergence than edge-emitting lasers. Even though at beam wavelengths greater than 10µm the output becomes multimoded, below LWIR surface-emitting geometry is a better choice [8].

Figure 2: (a) Semiconductor edge-emitting laser. (b) Surface emitting geometry (used in VEC- SELs). Source: [8]

3 VECSEL Design Considerations

3.1 Material System

Designing the VECSEL requires knowledge of what sort of wavelength we want to operate at. We have opted for MWIR, at a wavelength around 2.0µm (hence “2.xµm”). Referring to Figure 3, it seems that we can choose between GaSb and InAs as base substrates for our growth, since they are nearest to our desired emission wavelength1 . However, growing from InAs is impractical for numerous reasons: firstly the QWs that would need to be used in its active region would be GaInAsSb or InGaSb, which are crystallographically difficult to uniformly grow. Further, they are type II QWs which means that lasing from them is difficult to achieve (though not impossible at higher wavelengths), since carrier transfer in type II QWs is likely to invoke nonradiative recombination mechanisms. The differences between type I and II QWs are illustrated in Figure 4; in type I QWs the band offset is such that QWs have higher band energy than the barrier layers in the valence band, but lower band energy than the barrier layer in the conduction band. This means electrons and holes are mostly confined within the QW layer, and spatially direct recombination can occur easily. Conversely in type II QWs the QW has higher band energy than the barrier layer in both the conduction and valence bands. Thus, it is energetically preferable for electrons in type II QWs to be confined to the barrier layer of the material’s conduction band, while the holes are confined to the quantum well layer of the valence band (see Figure 4). This in turn results in little overlap between the electron and hole wave functions, a relatively low gain, and difficulty in obtaining lasing.

1 As a note, the correlation between Eg and emission λ is simply described: E = hν = hc/λ, so λ = hc/Eg.

4 Therefore our substrate material for our VECSEL growth will be GaSb. To achieve emission at our desired wavelength around 2µm we will alloy GaSb with In to create type I In0.2Ga0.8Sb QWs (ratios are approximate). Again referring to Figure 3, we can intuitively see why we have the 0.2:0.8 ratio; adding In to the GaSb lets the emission slide down the direct gap between GaSb and InSb, lowering the wells’ bandgap energy2 and thus raising their emission wavelength. We grow these QWs in Al0.25Ga0.75Sb barrier layers and AlSb cladding layers.

Figure 3: Bandgap energy (implies emission λ) and lattice constants of various III-V semiconductors at room temperature. The highlighted area represents the mid-infrared part of the e/m spectrum. Source: E.F. Schubert, Light Emitting Diodes (Cambridge Univ Press)

In fact, III-Sb VECSELs are currently the sole SDLs at and above the 2µm range. Their development has progressed to a max power of 6W CW, and 16W pulsed-pumped output [10]. Compared to the demonstrated power and wavelength performance of more mature VECSEL technologies (see Figure 5), this pales, but is still quite promising. The current power record for CW emission from any VECSEL (regardless of material system) is 106W, which was achieved by

2Note that the wells are actually split into various energy subbands (not denoted in Figure 4), with E α n2, analogous to how the diﬀerent energy levels available to a particle trapped in an inﬁnite potential well can be described by n2π2¯h2 En = 2ma2 Where a is the width of the well and m is the mass of the particle. However, the actual bandgap of the well is roughly equivalent to the ground-state energy (n=1) of the subbands, so we approximate the two.

5 Figure 4: Differences between type I and type II quantum wells. Note that while in type I QWs all carriers are confined to the QW region, in type II electrons are confined to the barrier region and hole are confined to the QW region. Bandgap energies modeled at 293K. Source: [9] going into a transverse multimode regime with M2 significantly greater than 1 at 1040nm3 [11]. A major factor in scaling the power of III-Sb VECSELs up to the level of InGaAs systems is understanding and addressing thermal issues unique to the III-Sb system.

Figure 5: Range of output wavelengths and powers demonstrated with VECSELs. Source: [12]

3.2 Top v. Bottom Emitting Designs

A major diﬀerence then, between III-Sb systems and previous SDLs, is the packaging techniques that can be used in each system to address heat concerns. There are two major designs - top and bottom emitting (see Figure 6), and they are both ways of addressing the same issue of thermal management. In a top emitting VECSEL, the DBR is grown lattice matched on top of the substrate, and then the active region is subsequently grown on top of the DBR. Single crystal diamond is placed in

3 M2 ratio: explanation of the M squared ratio requires an understanding of beam parameter products (BPP). A laser’s BPP is the product of its beam’s divergence angle and the radius of the beam at its narrowest point (ie: the beam waist). The BPP basically quantiﬁes the quality of a beam, and how well you can focus it on a given spot. An ideal Gaussian beam, TEM00, has the lowest possible BPP, λ/π. The ratio of your beam’s actual BPP to that of an Gaussian beam at your emission wavelength is your M2 ratio, which functions as a wavelength-independent measure of beam quality (which you obviously want to be as close to 1 as possible).

6 the cavity through capillary bonding to the active region to serve as the heat spreader. Putting the diamond so close to the active region is desirable since it reduces red shift and can more eﬀectively distribute the heat generated inside the active region to a water-pumped copper heat spreader. However, having the diamond in the way also runs the risk that it will absorb incident photons and introduce loss to the laser system. Further, the grade of diamond required is extremely cost- ineﬀective, especially when compared with the chemical vapor deposition (CVD) diamond used in the alternative bottom emitting designs.

Figure 6: Top and bottom emitting VECSEL designs. Note for bottom emitters the active region is actually grown on the substrate, but with an etch stop layer added in to alloy wet etching of the substrate oﬀ the active region (permitting lasing).

In bottom emitting VECSELs, the active region is grown on top of the substrate, then the DBR on top of that. The CVD diamond used as a heat spreader is then placed outside the cavity, onto the bottom of the DBR. For substrate removal in III-Sb systems, an etch stop layer is added between the substrate and the active region to allow etching. Although the technology for such layers is in its infancy in comparison to say the GaAs material system, adequate etch stop layers for III-Sb substrate removal have been developed [13]. The benefits of this design lie in that the diamond, being outside the cavity, does not introduce any optical loss to the lasing process. Moreover, the technique is cost-effective and historically, bottom emitting designs have reached the greatest power outputs [14]. The design we use for our growths is similar to the bottom emitting, but slightly different; we grow our DBR on our substrate, then the active region on top of it. We etch off most of our substrate, and then attach our chip with the DBR-side down to our heat sink. With our geometry it would be optimal to use optical grade CVD diamond heat spreader, soldered to our VECSEL’s DBR with a gold layer (because of gold’s high thermal conductivity). We would opt for diamond above a cheaper alternative like high-purity copper because copper only conducts heat at 400Wm−1K−1, while diamond conducts at 1800Wm−1K−1. The surface roughnesses of the two materials are more or less the same, meaning they can both provide smooth interfaces with the semiconductor device. However that whole arrangement is expensive; instead of using Au to connect the DBR and the heat sink, we use a thermally conductive adhesive paste with a Ag base. Further, instead of CVD diamond we will use a Cu-Ag-Al heat sink, pumping water through it to dissipate thermal energy. Although this arrangement will not dissipate heat at optimum efficiency, we are not attempting to scale our laser to extremely high powers, and thus while thermal management is still a major concern, we can afford to use a less intensive cooling system.

7 3.3 Importance of the Gain-Microcavity Alignment

But why are we concerned with thermal management? What happens if we overheat the device? Overheating of the VECSEL active region leads to thermal rollover, an effect that causes a decrease in output power, even when the pumping power is increased. This happens because without appropriate cooling systems, thermally excited carriers are more likely to escape QWs via nonradiative decay routes. Further, with increased temperature QW photoluminescence (PL) spectra red shift (the peak of the gain shifts to a higher wavelength). If this happened uniformly throughout the device, this would not matter, but the rate at which the gain and the micro-cavity resonance red-shift with temperature is slower than the shift in the QW PL spectrum (see Figure −1 7) [15] ; for the In0.2Ga0.8Sb QWs we use, the red shift occurs at about 0.3nmK , while our GaSb-AlAsSb DBR does at about 0.1nmK−1. Normally this effect is accounted for in the growth process by designing layers with an off-set; this allows the red-shift to occur up until the optimal operating temperature. An example of this effect is shown in Figure 7, where the gain-microcavity alignment is reached at about 80°C.

Figure 7: Reﬂectivity and edge photoluminescence spectra of a 2.25µm SDL structure for several heat sink temperature 20, 40, 60, and 80°C. Note that in the DBR’s stop-band, the absorption dip (the subcavity resonance) results from multiple reﬂections between the chip-to-air surface and the DBR, which cause a superposition of the QW absorption and the microcavity resonance. Note that it does not necessarily coincide with the resonant wavelength of the microcavity. The goal of the VECSEL grower is to match the subcavity resonance with the PL intensity’s peak to achieve gain-microcavity alignment. Source: [16]

4 Growth Process

4.1 Molecular Beam Epitaxy

Now that we have a basic background of VECSELs, how do we go about fabricating them? Recalling the basic structure of a VECSEL, we have a semiconductor chip, a heat sink, an output coupler, and a pump beam. The most critical component in the VECSEL’s function is the semiconductor chip - it contains the DBR, a highly reﬂective mirror that makes up half the laser’s

8 optical resonator, and the QW-based active region, which is required for the laser to have any gain. The semiconductor chip is grown through a process called molecular beam epitaxy (MBE). MBE is a technique in which you subject a heated crystal substrate (in our case, GaSb) to vaporized atomic or molecular beams in an ultra-high vacuum (UHV) environment4. The substrate is rotated to ensure uniform beam deposition, while its temperature is monitored with an optical pyrometer. At the right temperatures and flux rates, the atoms or molecules in the beam effectively are incorporated into the crystal lattice, allowing heteroexpitaxial growth of semiconductor compounds like GaAs, InP, GaSb, AlGaAs, etc. The MBE reactor used is a VG V80H, and we use Knudsen evaporation cells and gas-source crackers as beam sources (see Figure 8). The crackers are kept at 900°C to break down As4 and Sb4 to their dimer vapors As2 and Sb2, so that they can bond to the substrate lattice effectively. Apart from the group Vs, we have Al, Ga, and In source cells, along with GaTe for n-doping and Be for p-doping studies. Emission from these cells can be controlled with rapid-action mechanical shutters that interrupt the beam flux, allowing fine control of the growth structure to the degree of being able to deposit single-monolayers of a given epitaxial material. The growth rate in the reactor is about 1 monolayer per second, which corresponds to roughly 1µm/h.

Figure 8: MBE reactor schematic. Source: Henini, M., Thin Solid Films, 306, 331, 1997

On top of the precision we can obtain through MBE growth, given the UHV nature of the MBE system we can use reflection high energy electron diffraction (RHEED) analysis to monitor our growths in situ. Although RHEED is primarily used for calibrating the group III growth rates, it can also be used to understand the surface characteristics of the sample; for instance at the start of most growths the substrate needs to be heated to high temperatures to remove a pre-grown oxide layer from its surface. The RHEED diffraction pattern in this case goes from being murky with no RHEED pattern visible with the oxide layer present to being spotty with a visible but irregular RHEED pattern once the layer is removed. In effect, we can use RHEED to qualitatively characterize the surface during our growth, not only for oxide removal, but also in say interfacial misfit (IMF) layer growth and in determining if there are excesses of group IIIs or Vs on the samples surface. Further, if we did not have a reliable pyrometer we could use RHEED to monitor growth temperature; with GaSb, above 435°C there is a 1x3 pattern, while below 435°C it becomes a 2x5. 4UHV implies operation at or below 10−9 torr, or 10−7 Pa.

9 4.2 MBE Calibration

The main use for RHEED is group III flux rate calibration. The RHEED system is composed of an electron gun that emits high energy (about 10keV) electrons to be reflected off the substrate, so that when a phosphor screen is placed at the correct angle a diffraction pattern is observable. In fact, when the intensity of a single stationary spot in this pattern is analyzed with respect to time while the beam impinges the sample, the intensity varies sinusoidally, giving a resulting curve that resembles damped harmonic oscillations and correlates to mono-layer growth of the crystal (see Figure 9a). Applying a fast Fourier transform (FFT) to the curve can then be used to deduce the growth rate of incident group III atoms. These calibrations are then performed at various growth temperatures for a multiple-point calibration, yielding a growth rate chart (in the form of an Arrhenius plot) exemplified in figure 9b. This plot is then be used when growing samples at various temperatures to determine the growth rate for a given cell.

Figure 9: a. Oscillations of the amplitude of a RHEED point over time. The amplitude reaches a maximum when the surface is flat, which means that a single period in the oscillations corresponds to one monolayer of growth. Half-filled surfaces have maximum roughness, and so reflect diffusely, leading to a minimum of reflected intensity. Source: [17] b. Exemplary Arrhenius plot of the logarithm of the rate constant as a function of temperature. Note that the different plots on the chart represent different calibration runs for the cell; the single line separated from the group is from a recent calibration after having refilled the Ga cell.

5 Characterizing the Semiconductor Chip

5.1 Chip Structure

We use a (100) oriented GaSb substrate for growing VECSEL structures in MBE. We then grow 19 LM AlAs0.08Sb0.92/GaSb pairs to the substrate, with GaSb layers 132nm thick and AlAs0.08Sb0.92 layers 153.8nm thick (thickness derivation to be explained), corresponding to a com- plete DBR thickness of about 5.41µm. We then grow In0.2Ga0.8Sb QW layers between Al0.25Ga0.75Sb barrier layers (repeating the process 9 times), all between two AlSb cladding layers (see Figure 4

10 for energy diagram) for a total active region thickness between 2 and 3 microns. We finally add an approximately 5nm thick GaSb cap layer to prevent oxidation of the AlSb top cladding layer. We wet-etch off most of the GaSb substrate at the end, leaving roughly 100µm of it on, to make handling the sample easier (since handling a 10µm wafer would be quite difficult). See Figure 10 for diagram.

Figure 10: Rough schematic of the epitaxial structure of the semiconductor chip.

5.2 Characterization of DBR

To grasp how we characterize the DBR, it is first important to understand its function, and then what factors affect that function. The DBR is a multilayer on-chip mirror that completes half of the laser cavity. The mirrors reflectivity in its stop-band (the width of wavelengths it is intended to reflect) must then be on the order of 99.9%, since the single-pass gain through the active region is only a few percent [18]. The structure itself can be viewed as a periodic structure of a layered medium composed of N layered pairs of two different semiconductor materials with varying refractive indices nlow and nhigh. In our case, we use GaSb (n ≈ 3.79) and AlAs0.08Sb0.92 (n ≈ 3.25)5 layered pairs. We add As to AlSb to lower its lattice constant, allowing lattice matching between GaSb and AlAsSb6. Each layer interface causes partial reflection of incident light waves, so the entire structure works by cumulatively building a constructive interference pattern from the combined reflections. Each layer must then reflect in phase with the others, meaning that the layer thickness should be a quarter of the wavelength of the light passing through the DBR so that φ, the phase difference between light waves, = π. Theoretically modeling the reflectivity of and electric field patterns within a DBR is usually done using a transfer matrix method [20], which can be simplified to model DBRs as fixed hard mirrors with an effective reflectivity RDBR and bandwidth ∆λDBR given by [20] [21]:

5Note that these values are approximate; refractive indices for materials change both with wavelength and temperature. Full characterization of our DBR’s refractive indices would require a photomodulation reﬂectance study to be performed [19]. We can calculate theoretical values for the refractive indices based on the position of the center of the observed stop-band and the widths of the layers we grew - if we take the stop-band center λ0 to be 2.0µm, and combine that with our requirement of λ/4 = d for the DBR layers, we determine our values for n from λ = λ0/n when given d, the thickness of our layers. 6Lattice matching refers to aligning two dissimilar materials with close lattice (recall Figure 3). It is required in crystal growth to reduce propagation of defects in the crystal’s structure. In relatively thick structures like DBRs this is critical, since defect propagation will hinder the eﬀectiveness of the device. In structures like the QW-arrangements in our active regions, LMing is also required to align the relative positions of the conduction and valence bands [16].

11 2N !2 1 − (n1/nE)(nlow/nhigh) RDBR = 2N (1) 1 + (n1/nE)(nlow/nhigh) ∆λ 4 1 − (n /n ) 2 ∆n 2πc DBR = arcsin low high ≈ ω , when ω = (2) λc π 1 + (nlow/nhigh) π n λc

Where n1/ne is the incident to exit refractive index ratio (in our case simplifies to 1 since the light both enters from and leaves to air), ω is the center angular frequency of the reflection band, and λc is the wavelength of the light (2.0µm). Using (1) to calculate the peak reflectivity of the DBR with the previously given values for nlow and nhigh yields RDBR ≈ 0.999, and likewise, using (2) to find the bandwidth yields ∆λDBR ≈ 264nm. This is a fine width for a stop-band; if we were attempting to tune our laser, we would need to consider increasing the refractive index difference between our DBR layers to allow for greater tuning possibilities, but that is beyond the scope of our work.

Figure 11: DBR Schematic. E0+ and E0− respectively correspond to the light input and output from the DBR. d1 and d2 are the thicknesses of each layer, λ1 and λ2 the wavelengths of light in them (which is of course changed by the layer type). Source: [20]

To actually characterize the structures once they have been grown, we use a combination of scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) based temperature dependent reflectivity (TDR). For the SEM scans, the DBR is cleaved and then a cross-section of it is examined to ensure layer uniformity (see Figure 12a). However, this approach is fairly qualitative - to get quantitative data on the reflectivity and stop-band of the DBR (the characteristics that matter the most to us) we employ FTIR. FTIR spectroscopy obtains an IR spectrum of absorption, emission, or photoconductivity of a material. Essentially the process involves using a broadband light source to shine a beam of many frequencies at your sample simultaneously, then modifying the beam with a Michelson interferometer to make the beam contain a different combination of frequencies, all while measuring how much of the beam is absorbed by the sample. Working backwards and employing a Fourier transform then yields a graph of reflectivity as a function of wavelength, as shown in Figure 12b. There are further characterization techniques that can be used on DBR structures: transmission electron microscopy (TEM) can provide the same functionality as SEM, Nomarski imaging can provide a rough idea of surface quality while atomic force microscopy

12 (AFM) can provide a closer resolution look at the surface, and x-ray diﬀraction scans (XRD) can be used to determine the composition of the DBR layers and to understand how well the layers are lattice matched. See appendix for related scans on the DBR we ended up using.

Figure 12: (a) SEM image of the cross-section of a DBR note this image is only useful in the context of analyzing layer uniformity. (b) TDR scan for reﬂectivity as a function of wavelength in our GaSb-AlAsSb DBR (grown on a sample denoted “R12-30”). Note that the stop-band’s center λ0 is a little shy of the 2000nm we are aiming for, but our predicted bandwidth of about 270nm is roughly correct, as was our prediction of the extremely high reﬂectivity in the stop-band.

5.3 Characterization of Active Region

5.3.1 Active Region Function

As with the DBR, we must first understand the function of the active region before we can understand how it is characterized. The active region provides the gain for the laser, and ours uses QW layers to such effect. QWs, as evident in Figure 4, are discrete potential wells created by embedding a material of low bandgap energy (for us In0.2Ga0.8Sb) into a barrier layer of higher energy (Al0.25Ga0.75Sb). Most of their quantum-properties derive from their ability to confine charge carriers (electrons and holes) from the normal three dimensions of space down to two. They are usually less than 10nm (40 atomic layers) thick, which means that their emission transition energy is set not only by their bandgap, but also by slight variations in their thickness and their barrier layers bandgap. Optical emission occurs between the lowest level of the conduction band and the highest level of the valence band, and its exact energy can be calculated via k·p analysis [21]. The material gain gth required to reach threshold and overcome absorption losses can be calculated by [22]:

1 1 1 gth = α + ln (3) NwΓwξ 2L RDBRROC

13 7 Where Nw is the number of QWs (=9), Γw the average optical confinement factor per QW , ξ is the energy confinement factor (=2), L is the cavity length (≈40cm), RDBR and ROC are the reflectivities of the DBR and output coupler (0.999 and about 0.96, depending on the output coupler selected). Somewhat counterintuitively, increasing the number of QWs in the structure lowers the material gain required for lasing more wells give more gain, and so help overcome losses and give more output power. Thus we want as many wells in our structure as is crystallographically sound, while keeping in mind that too many could result in excess strain buildup which would hinder laser performance. Most VECSEL structures have anywhere between about 6 and 12 QW layers in their active regions - ours is grown with 9.

5.3.2 QW RPG Structure

Figure 13: Refractive index n and electric ﬁeld |E|2 of a 12 well RPG sample. Note that only four of the 30 bottom DBR pairs are shown. Source: [24]

But then what sort of arrangement are the QWs placed in? Recall that back-reflection between the DBR and the chip-air interface causes microcavity resonance with a standing electrical field inside the active region. The QWs themselves are then separated in a resonant periodic gain (RPG) arrangement, which places them at the antinodes of the longitudinal standing electrical field, ie: 8 with a separation of λlaser/2 (see Figure 13) . This enhances the field intensity to a factor of ΓRP G = 2 in comparison with the average intensity [26]. An apt analogy for the placement of the wells is considering the equivalent harmonic motion of pushing a child on a swing set - you

7 In VCSELs (essentially sandwiched VECSELs, with the output coupling mirror stuck to the active region) Γw is fairly easy to calculate divide the 3D integral of the intracavity laser ﬁeld in the active region by the 3D integral of the intracavity laser ﬁeld over a single QW [23]:

RRR |E|2dxdydz ν Γ = active ≈ active w RRR 2 QW |E| dxdydz νQW

Where νactive and νQW are the volumes of the active region and QW. However in VECSELs, the electric field extends beyond the end of the active region into the cavity thus simulating and integrating over the fields becomes a sizeable problem beyond the scope of this paper, given the size of the VECSELs whole cavity compared to the VCSELs microcavity. 8Note that the refractive index on the y-axis of the figure can also be interpreted in terms of bandgap energy 4 since n Eg ≈ 95eV (Moss relation) [25], so index of refraction and bandgap are roughly inversely proportional.

14 push the child at the antinode of their motion to maximize the effectiveness of the force you exert, while if you push them at the node of their motion (ie: the end of the swing), pushing them would slow them down. The QWs provide the extra push needed to optimize gain of the SDL. The improvement in the gain is then caused by the enhanced interaction between the carriers confined within the QWs and the electric field of the cavity’s standing mode9.

5.3.3 PL and XRD Characterization

When characterizing the active region, we are then concerned with what wavelength it emits at, how much strain exists in it, what kind of composition the QWs have, and how broad their emission curve will be. We can determine these characteristics through XRD and PL scans. Photoluminescence refers to when a surface absorbs photons and then reradiates photons. This process can be explained by the model of above-bandgap photons exciting electron-hole pairs, and then the pairs recombining and emitting photons. In our PL setup then (see Figure 14a), we pass an IR-laser through a chopper (to bring the broadband beam down to a single frequency, and reduce noise from the room) and shine its beam onto our sample active region [28]. The resulting emitted radiation is then collimated with a collecting lens and focused into a monochromator. The monochromator then scans for individual wavelengths by using bent reflectors to disperse input light. Those individual wavelengths are passed to a photomultiplier or photodetector, which measures the intensity of the light at a given frequency. Then by altering the monochromator’s configuration we can obtain the intensity of the entire spectrum of IR light over which our active region is emitting. Ultimately, our PL emission spectrum resembles Figure 15. Note that the peak emission wavelength (about 1940nm) is just shy of the 2µm we are aiming for, but that should be expected since the PL measurements were taken at room temperature, and we are accounting for the previously described gain-microcavity offset. Further, it is important that the emission curve is not very broad - just shy of 100nm in width10. This leaves relatively little room for tuning the laser (if a wider curve was desired, growing a QD or QDash active region would be preferable). However this is optimal when attempting to scale to high powers, since it means most of the active region’s emission is concentrated at a single wavelength. PL scans of our active region are in the appendix. PL is then used in conjunction with symmetric (004-oriented) and asymmetric (115-oriented) x-ray diffraction (XRD) scans to determine the relaxation and strain of the structure, along with the composition percentages of the semiconductor materials inside of it. In XRD scans (see Figure 14b), an x-ray tube shines a broad-spectrum x-ray beam through a monochromator onto the sample, while rotating the sample around its ω-axis.

9Just as a note, this RPG arrangement is a relatively recent innovation; traditional VCSEL and VECSEL designs cluster QWs at the antinodes of the electric field (the 4x3 arrangement), but this design can contribute to strain and decrease thermal efficiency [27] 10It is also important to understand that the PL width is a function of carrier distribution energy subbands within the QWs have slightly different energy levels, so recombination from the different subbands results in photon emission at slightly different wavelengths, with the most frequent recombination energy being the peak of the PL curve, and then less frequent ones being the rest of the distribution.

15 Figure 14: (a) PL schematic (b) XRD schematic Source: [28]

The beam is then reflected off the sample, and diffracted intensity is measured by a scintillation detector as a function of the specimen angle, ω. The widths and relative locations of the resulting scans, exemplified in Figure 16, can then be used with modeling software both to calculate the percentage composition of the materials in the QWs (confirming the RHEED calibration results) and also to determine the strain in the material.

Figure 15: Exemplary PL emission spectrum for QW-based active region.

Figure 16: Exemplary GaAs VECSEL XRD. GaSb XRD looks nearly identical (even spacing of QWs, compressive strain), except if symmetric and asymmetric scans were used in conjunction they would yield diﬀerent compositions than the example ﬁgure.

16 6 Lasing Setup

Figure 17: Optical setup for lasing. Labeled components: (A) pump laser fiber, coming in from a 75W diode laser at 808nm with a beam diameter of 400µm (B) focusing optics, effectively a Keplerian telescope that concentrates pump beam on single spot of mounted sample (C) Cu-Ag-Al heat sink, water-cooled (D) mounting spot for the sample, stuck to the Al of the heat sink (E) stack of razors, to block any reflection off the sample (F) laser cavity (G) focusing lens (H) output coupler, about 40cm from sample (I) photodetector, measures output power from sample.

Temperature of the heat sink is monitored with a Newport Model 3150 temperature controller, output power of the pump beam is controlled with a Newport Model 5600 laser diode driver. Assuming we have a good sample, an aligned cavity and functioning optics the above setup should be fully capable of lasing with the GaSb-based VECSEL structures we have grown and characterized. However, we ran short on time when attempting to obtain lasing action - along with the time required to grow and characterize our chip samples so that they were adequate structures for lasing, assembling the lasing setup (aligning the laser cavity, machining together a heat sink from available materials, ensuring the cooling systems were functional, setting up and testing various pump beams, assembling and aligning the focusing optics, etc.) took up the majority of the time available for me personally to work on this study. Fortunately, VECSEL chips with similar parameters to those described and characterized in previous sections of my report had previously been investigated by my group. The results from lasing in these VECSELs are detailed in the following section as an exercise in understanding what sort of characteristics are expected from lasing with the structures that we grew over my time on this project.

17 7 Lasing Results

Once CW lasing action is obtained with a VECSEL sample, some of the characteristics we are concerned with include: its potential for power scaling, how its emission spectrum shifts with pump power, the differences in emission between the edge and the surface of the chip, and finally how the chip performs under pulsed pumping conditions. For power scaling, incident pump power is increased on the chip sample either until thermal rollover occurs (visible by a sharp dip in output power with increased pump power) or until it reaches saturation point (visible by output power remaining constant with increased pump power). The pump beam diameter is then altered (recall that output power correlates to gain area, and thus pump beam diameter), with the largest pump beam diameter expected to give the greatest output power. Since the focusing optics for the pump beam are effectively arranged in a Keplerian telescope setup, shrinking the spot the size is a matter of increasing the focusing lens’ focal length11, while increasing the spot size can simply be done by horizontally shifting the telescope setup with respect to the sample. In our example sample R11-80 (see Figure 18) a maximum output power of about 1W is achieved with a spot diameter of 460µm and a pump beam power of about 15W, when using an output coupler with 6% transmission12. Note that for the 460 micron spot, thermal rollover is never reached; the sample ultimately ended up reaching about 3W of output power with higher pump beam power. Observing how the emission spectrum of the sample shifts with pump beam power on Figure 19, we note that the spectrum shifts with dλ/dP = 2.5 to 3.0 nmW−1. The discrepancy between figures 18 and 19 (the maximum power in 18 is obtained at around 5W, while in 19 it is at 3.1W) is a result of taking the measurements at different spots on the sample.

Figure 18: Power scaling on R11-80 sample. Note that the sample is (in terms of epitaxial structure) near-identical to previously characterized samples.

11 Since dspot = dpump ∗ f2/f1 12Note that this is a high transmission rate intended to provide maximal power, at the cost of raising the threshold intensity required to obtain lasing. At the beginning of the tests, when attempting to just achieve lasing, an OC with a lower transmission rate (usually about 1%) is used instead, since it lowers the threshold intensity and thus makes it easier to obtain any lasing

18 Figure 19: Redshift of laser emission spectrum as a function of pump power, with a spot diameter of 330µm.

Beyond the power scaling and the emission spectrums redshift, it is also important to take note of the differences between the surface and edge PL on the chip, as shown on figure 20. The VECSEL is designed to be a surface emitting chip, however on the edges leakage happens when total internal reflection from non-normal emission of QWs gets internally reflected through the active region layer (in approximately the same manner as a waveguide or a fiber) to the edges of the sample. Thus the edge PL spectrum is broadband, instead of being blocked off by the DBR mirror at designated stopbands, it exhibits the wide curve seen in figure 20, where the shoulder at around 1700nm is accounted for by the GaSb PL peak.

Figure 20: Surface and edge PL of R11-80. Note for the surface PL that the second gain peak is just the PL transmission through the lower wavelength part of the DBR.

19 8 Conclusion

We have taken a GaSb substrate and epitaxially grown a 2.x µm VECSEL structure on it with MBE (after performing a multi-point RHEED calibration on the MBE source cells). We proceeded to characterize the primary components of the semiconductor chip (the DBR, the active region) with methods including XRD, PL, SEM, TDR, FTIR, and Nomarski scans. We then assembled an optical setup to attempt lasing in the samples we grew, but failed to achieve lasing due to time constraints. However, previous research provided a framework through which we could consider the types of results expected from our work. The elementary theories behind the VECSEL concept and growth process have been detailed in this report, as have the results obtained over the course of my time on this project. Potential future work on this project would include taking more time to obtain lasing in the previously grown samples. Expanding beyond the scope of this paper, further work in the ﬁeld includes expanding III-Sb systems to higher wavelengths and powers, along with researching new ways to incorporate VECSEL technology into optoelectronic devices.

9 Acknowledgements

This research was performed at the University of New Mexico’s Center for High Technology Materials. It was funded in the “REU Site: Nanophotonics @ UNM” program under NSF Grant EEC-1063142. I would like to thank Dr. G. Balakrishnan and C. Hains for their invaluable assistance in performing this research, along with my graduate student mentors S. P. R. Clark and P. Ahirwar. I would like to extent further thanks to REU Program Director Dr. M. Osinski, and Coordinator L. Bugge.

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Updated August 3rd 2012.

22 10 Appendix

Figure 21: Developed lasers in the electromagnetic spectrum. http://upload.wikimedia.org/ wikipedia/commons/4/48/Commercial_laser_lines.svg , last accessed 03.08.2012

23 DBR Characterization The chip we ultimately attempted to use for lasing (“R12-35”) was grown on a DBR named R12-30.

Figure 22: Nomarski scan for R12-30 DBR surface; note there is no cross-hatching of the surface, and there are very minimal surface features that means the sample has minimal strain and few defects.

Figure 23: XRD scan for R12-30 DBR; note the peak in diﬀraction intensity on the right is the GaSb peak, and that on the left is the AlAsSb peak. The diﬀerence between their angles (388 arcsecs) is close enough to constitute lattice matching in a DBR structure (although if it was in the barrier layers of an active region, it would need to be closer to 200 arcsecs).

24 Active Region Characterization

Figure 24: PL scan for R12-35 VECSEL. Note the second gain peak is just the PL transmission through the lower λ part of the Bragg reﬂectivity (recall that beyond the stopband there is a sudden dip in reﬂectivity).

Figure 25: Surface PL of the GaSb VECSEL from Figure 24 imposed on microcavity red-shift for sample. Note this laser reaches optimal alignment around the 2010nm mark at about 340K.