Quantum Wells Feature Quantum wells and superlattices come of age Zh.l. Alferov A.F. loffe Physico-Technical Institute I Heterostructure quantum wells and superlattices have a pivotal role in the history of compound semi- conductors. This short review looks at the underlying physics, as well as the applications of quantum wells and superlattices. It starts by looking generally at the major developments in the field, before look- ing specifically at the advances made at the A.F. Ioffe Physico-Technical Institute in Russia. t he development of the its use in preparing room tempera- parabolic band would break into double-heterostructure (DH) tureAIGaAs DHS lasers in 1977. mini-bands separated by small for- T laser is a sensible starting Low dimensional semiconduc- bidden gaps and having Brillouin point for looking at the history of tor structures have formed a major zones determined by this period. heterostructure quantum wells. new branch of physics research. Similar ideas were described much Owing to electron confinement in Structures with such a small scale earlier by L.V. Keldysh in 1962 [3] DH, the double-heterostructure in one or two spatial dimensions, when considering the periodic po- laser became an important precur- their electronic properties are sig- tential produced on semiconduc- sor of in the development of quan- nificantly different from the same tor surface by intense ultrasonic tum well structures. This results material in its bulk form. These wave. from the fact that when the mid- properties are changed by quan- At the Physico-Technical dle-layer of these lasers had a thick- tum effects. A quantum well in Institute in St Petersburg in the ness of some hundred angstroms, which chargecarriers are restricted then Soviet Union, R. Kazarinov the electron levels would split to moving in two dimensions is an and R. Suris considered the theory because of the quantum-size example of such a structure. of current flow in superlattice effect. A clear manifestation of the structures in the beginning of the Further, the development of quantum-size effect in the optical 1970s [4].Their work revealed that epitaxial growth techniques for spectra of GaAs-AIGaAs semicon- the current between wells was de- these structures brought with it ductor heterostructures with ultra- termined by tunnelling through the ability to fabricate high-purity thin GaAs layer (quantum wells) the potential barriers separating and ultrathin layers that were to was demonstrated by Raymond the wells. It also predicted very im- become very important for quan- Dingle et al. in 1974 Ill. The au- portant phenomena: tunnelling un- tum wells. Two main methods of thors observed a characteristic der electric field when the ground growth with very precise control step-like behaviour in absorption state of a well coincides with an of thickness, planarity and other spectra and systematic shifts of the excited state of the next well and parameters were developed in characteristic energies with a stimulated emission resulting from 1970s -- molecular beam epitaxy quantum well width decrease. photon-assisted tunnelling be- (MBE) and metal organic chemical Experimental studies of super- tween the ground state of one well vapour deposition (MOCVD). MBE lattices, meanwhile, started after and excited state of a neighbour- was the first method to be of prac- the initial work of IBM scientists ing well, which is lower by the en- tical importance for III-V het- L.Esaki and R.Tsu in 1970 [2]. ergy due to applied electric field. erostructure technology, following These researchers considered elec- At that time L. Esaki and R. Tsu the pioneering work in the begin- tron transport in a superlattice i.e. independently considered reso- ning of 1970s by A. Cho at Bell at an additional periodic potential nant tunnelling in superlattice Laboratories. Not long after created by doping or changing the structures. MOCVD, which had its origins in composition of semiconductor ma- the early work of H. Manasevit, terials with the period bigger, but Superlattice structures found broad application in III-V comparable with, the lattice con- heterostructure research after stant of the crystal. In what Leo Pioneering experimental studies of R.Dupuis and PDapkus reported Esaki called a 'man-made crystal', a the superlattice structures were IIl-Vs Review• Vol.lO No. 7 1997 26 I I I I 80 lnGaAsP/GaAs W = 100 ~m k = 0.8 ~tm CW L= 1.2mm 70 NE 2 60 g 3 50 40 1--45-A757 2--3-A799 30 E // ] I I I I 20 0 1 2 3 4 5 1 (A) Figure 1: The CW light-current characteristics of InGaAsP/GaAs separate-confinement single quantum well DHS laser diodes. In (1), the diode had high and low reflective coatings, while in (2) the diode had a high reflective coating only. carried out by L.Esaki and R.Tsu very low concentration of defects. with the superlattices grown by Many years later in 1983, after vapour phase epitaxy (VPE) in a G.Osbourn's theoretical study at GaPxAsl x/GaAs system. At the Sandia lab and the first successful same time, in our own laboratory, preparation of a high quality the first multi-chamber apparatus strained-layer superlattice (GaAs/ for VPE was developed. As men- In0.2Ga0.sAs by M.Ludowise at tioned before, this was used to pre- Varian), N. Holonyak at the pare the superlattice structure University of Illinois achieved the GaP0.3As0.7/GaAs with the thick- first continuous wave (CW) room ness of each layer 100 A and total temperature laser action using number of the layers of 200. these structures. From research Observed peculiarities of the volt- such as this it became clear that in your age-current characteristics, their strained-layer superlattice the lat- temperature dependence and tice strain becomes an additional photoconductivity were explained degree of freedom.This meant that by the splitting of the conduc- by varying the layer thicknesses tion band due to the one-dimen- and compositions it was possible sional periodic potential of the to vary, continuously and indepen- superlattice. dently, the forbidden gap, lattice These first superlattices were constant and other parameters of also the first strained-layer super- the overall superlattice. lattices. E. Blakeslee and J. At the beginning of 1970s Matthews, who were working with L.Esaki and his co-workers started L. Esaki and R.Tsu at IBM, succeed- using MBE to study the AIGaAs sys- ed in the mid-1970s in growing tem resulting in the submission of strained-layer superlattices with a a paper in March 1974 to Physical Fax +46 46 t68g 81 WWW httpd/www.epigreSS.se Quantum Wells Feature Review on resonant tunnelling -- up to 420 GHz were reported in a application of the high-mobility, the First experimental demonstra- GaAs resonant tunnel diode at two-dimensional electron gas for tion of quantum well heterostruc- room temperature. microwave amplification. In 1980 ture physics. In the study, they The restriction of the motion of this led to new types of transistors measured the tunnelling current the electrons to two dimensions in based on single n-AIGaAs/n-GaAs and conductance as a function of field effect transistors has long modulation-doped heterostructure applied voltage in GaAs-Ga&lAs been recognized and was First veri- being developed.The development double barriers and found current fied, for the trapped electrons in occurred almost simultaneously in maxima associated with this reso- inversion layers, in a magneto-con- France, where they were labelled nant tunnelling. Later in the same ductance experiment conducted TEGFETs (two dimensional elec- year L.Esaki and L.L.Chang ob- by A.B.Fowler et al.. in 1,966. tron gas FETs), and in Japan, where served resonant tunnelling in a su- Spectral effects due to spatial they became known HEMTs (high perlattice. This strong interest in quantization were observed in thin electron mobility transistor). resonant tunnelling was obviously bismuth Films in 1968 by V.N. The first quantum well laser op- connected with its potential appli- Lutskii and L.A. Kulik at the eration was demonstrated by J.P. cations in high-speed electronics. Moscow Radio-Electronics Institute. van der Ziel et al.. in 1975, al- The success of these endeavours is Pioneering work on modula- though the parameters of the las- seen in the fact that by the end of tion-doped superlattices [5] ing were much worse than for the 1980s picosecond operation demonstrated a mobility enhance- average DHS lasers. By 1978, how- had been achieved in a double res- ment with respect to the bulk crys- ever, R. Dupuis and P.Dapkus, in onant tunnel diode and oscillations tal. This stimulated research on collaboration with N. Holonyak, were reporting quantum well lasers with parameters comparable (a) with their conventional DHS coun- terparts. Their paper was also no- table for its specific use of the • :."',/,i,~,~" ~ x",~: ,,, " ,,, term 'quantum well'. Even with this advance, it was not until 1982 that the real advantages of quan- -~ ,,~ tum well lasers were demonstrat- ed. At that time, W.TTsang at Bell Telephone Labs used the many im- provements of MBE growth tech- t=O nology, as well as an optimized (b) structure (GRIN SCH), to achieve threshold currents as low as 160 A. cm 2. The idea of stimulated emission in superlattices that had been pub- , ..: , •. lished by R.Kazarinov and R.Suris [4] back in 1971, was Fmally real- ized nearly a quarter of century lat- er after a proposal by Federico t = 15 min t = 6 min Capasso. The proposed structure was strongly improved and a cas- ~ .~ , ,: • ... ,!:.~,: :.:::. ;.': ,~: ::. -.... cade laser developed by E Capasso gave rise to the new generation of unipolar lasers operating in the ~,.