Superlattices and Microstructures, Vol. 3, No. 3, 1987
MBE GROWTH OF
WELL AND SUPERLATTICE
CHEN Zonggui, SUN Dianzhau, LIANG Jiben, XU Zhongying HUANG Yunheng, GE Weikun and KONG Meiying Institute of Semiconductors, Academia Sinica Beijing, China (Received 18 August 1986)
MBE is the most highly developed technique to prepare ultra-thir~ multilayer heterostructures. In this paper, we report that high-quality quantum well and superlattice structures with controllable layer thicknesses as thin as one monolayer and with ideally abrupt GaAs/A2xGal_xAS interfaces have been obtained by a home-made MBE system and investigated from the view-point of crystallography and optical properties.
I. Experiment The MBE system consists of a growth chamber, an air lock and a pumping system. The b a ~ pressures prior to growth are below 2xl0Torr. The growth chamber is equipped with a manipulator, molecular beam source (six effusion cells with PBN crucibles), RHEED (10kV-5OkV), cylindrical mirror analyzer, quadrupole mass spectrometer and an ion gauge used for measuring molecular beam fluxes. All of the component parts of our MBE system were made in China. The source materials were Ga, A2, and As of 6 nines purity. The samples were grown under As-rich growth conditions at a temperature of 660-680°C. A 0.2-0.3 #m GaAs buffer layer was grown first on a Cr-doped semi-insulating (I00) GaAs substrate, followed by alternating GaAs well layers and A2GaAs(A2As) barrier layers with precisely controlled thicknesses. Most MQW structures were cladded by a 0.1-0.2 ~m A~xGal_xAS confined layer. The A2As composition in A2xGal_xAS varied from x=0.25 to 0.4. 2. Measurements 2.1 X-ray Double Crystal Diffractometry X-ray double crystal diffractometry is a nondestructive analytical technique that has been used to provide precise determination of layer thicknesses, crystal quality and uniformity of superlattice structures. Fig. I is an X-ray rocking curve measured from a GaAs/A2As superlattice structure, and exhibits several small satellite peaks around a main reflection peak. The superlattice period (D=372A), potential-well layer thickness (Lz-147~) and barrier layer thickness (Lb=225A) were calculated from the angular separation (AS) between peaks by the following formulas [1,2]:
0749-6036/87/030325 + 04 $0200/0
D = ~/2[cos~ 0 -
L b = DASO,s / ASA2As
L z = D - Lb, where we have i - 1.5405A(Cu Kel) with 80 = 33"02o (the GaAs 004 diffraction angle). From the X-ray rocking curve, it also can be seen that the main reflection peak has its FWHM comparable with that of the substrate, showing good crystal quality and excellent uniformity in layer thickness. 2.2
Photoluminescence (PL) has been proved to be a powerful technique to assess the QW material. In our experiments, the excitation source for the PL measurement is a mode-locked Ar + laser with = 5145A emission. Fig. 2(a) shows a typical PL spectrum of a MQW structure which has 5 quantum wells of Lz~I41A and a single well of Lz-19OA. For the MQW n=l transition, both electron-heavy hole recombination (peaking at Elh-l.535 eV) and electron-light hole recombination (peaking at: E12zl.5418 eV) can be identified. The corresponding peaks for the single well are denoted as Elh' ~ 1.526 eV and El2 = 1.533 eV respectively. The peak positions are very close to those calculated from the initially projected well width by the following equation: (2ml*E)i/2
= [2m2- (V0-E)] I/2 where V 0 is the barrier height, E is the electron energy above the GaAs conduction band edge or the hole energy below the valence band
© 1987 Academic Press Inc. (London) Limited
Superlattices and Microstructures, Vol. 3, No. 3, 1987 I
3 3 .O
8 Fig. I. 004 diffraction rocking curve of GaAs/A2As superlattice.
edge, ml* and m2* are the effective masses of the corresponding carriers in GaAs and in A2xGal_xAS respectively. It is noticed that the FWHM of the Elh line is as narrow as 1.7 meV at 12°K and varies slowly with increasing temperature as shown in Fig. 2(b). For comparison, the FWHM of the bulk GaAs band edge emission and its variation with temperature are also shown. It is obvious that the narrow line width and its insensitivity to temperature are good evidence for high quality MQW material. Furthermore, Fig. 2(c) shows that the variation of Elh is closely following the variation of the GaAs bandgap E as temperature increases, which implies that g the emission continues to be excitonic, which is also a characteristic of a high quality quantum well. For the sake of clarity, the temperature dependence of Elh for a poor quality MQW (sample 2) is also shown in Fig. 2(c). There are two regions of the line, both closely following the variation of Eg, but the differences AE between Elh and E in the two parts are different. In fact the va~ue of AE is larger at high temperature than at low temperature by an amount equal to the exciton binding energy. This shows that for a poor-quality MQW the radiative recombination is excitonic at low temperatures but band-to-band
Fig. 2(a). Photoluminescenee spectrum of a GaAs/A~GaAs MQW with different well widths at 12°K. (See text.)
T (K) Fig. 2(b). The variation of the FWHM of MQW and bulk GaAs band edge emission with temperature.
Superlattices and Microstructures, VoL 3, No. 3, 1987 I
> 1.50 r
T(K) Fig. 2(c). The variation of Elh (sample i) and GaAs bandgap E~ with temperature. The top line shows the temperature dependence of a
corresponding Elh for a poor quality MQW (sample 2).
Energy (eV) 1.90
E '~E ,,
F'?h / r°
/~J i XlO0
Fig. GaAs/A2GaAs pulses.
o7 5 0 0
Photoluminescence spectrum of a MQW excited by 200 ps Ar + laser
Super/attices and Microstructures, Vo/. 3, No. 3, 1987
Lz+2aT q r-L z GaAs/AIGaAs M 20K >, ,m c(D
B-B (e . k /
peaks in the higher energy region. Their energetic positions at 1.703 eV and 1.890 eV are attributed to the n=2 and n=3 electron-heavy-hole transitions E2h and E3h respectively. The energies for the n=2,3 electron-heavy-hole transitions calculated based on a rectangular-well model are close to the energetic positions of the spectral peaks. This indicates that the basic theoretical assumption, namely ideally abrupt A2xGal_xAS barriers, is well-fulfilled for our quantum wells. A fine structure of the luminescence spectrum for a GaAs/A2xGal_xAS quantum well (Lz=8.7nm) is shown in Fig. 4. A splitting of the dominant peak is obviously observed. The value of the splitting corresponds to the calculated energy shift introduced by a two-monolayer difference in the well width.
Energy (eV) Fig. 4. The fine structure of photoluminescence spectrum of GaAs/A2GaAs MQW with a monolayer well size fluctuation at 12 K.
at high temperatures, while for a high-quality well the recombination is excitonic at both low and high temperatures. Under the excitation of 200ps laser pulses from a mode-locked Ar + laser, transitions involving higher-energy states (n=2,3) in the quantum well have been observed. Fig. 3 shows the total low temperature (10K) photoluminescence spectrum taken from a GaAs/A2wGal.xAS MQW structure with well width Lz=90 A- Besides the dominant peak which is attributed to the n=l electron-heavy-hole transition Elh=l.537 eV, there are two weak
High optical quality MQW and superlattice structures with controllable layer thicknesses as thin as one monolayer and with abrupt GaAs/A2xGal_xAS interfaces exhibiting monolayer well-size fluctuations have been obtained by a home-made MBE system. In the meantime, MQW lasers have been successfully prepared.
Acknowledgement--The authors are thankful to Prof. K. Huang, Prof. L. Y. lin and Prof. C. M. Wang for support and encouragement, to Mr. Y. T. Wang for X-ray double crystal diffractometry measurements and to Mr. Y. P. Zhen for technical assistance.
REFERENCES [i] A. Gossard, Inst. Phys. Conf. Ser. 69, i (1983).  J. Matsui, J. Electrochem. Soc. 126, 664 (1979).  B. Deveaud, J. Y. Emery, A. Chomette, B. Lambert, and A. Regreny, Superlattices and Microstructures 2, 205 (1985).