GaAs heterostructures

Superlattices and Microstructures, Vol. 6, No. 4, 1989

391

PHOTOREFLECTANCE OF SELECTIVELY DOPED n-AIGaAs/GaAs HETEROSTRUCTURES

Y.S.Tang and Y.W.Xu University of Science and Technology of China, Hefei, China D.S.Jiang, W.H.Zhuang and M.Y.Kong Institute of Semiconductors, Academia Sinica, Beijing, China National Laboratory for Superlattices and Related Microstructures, Beijing, China

(Received August 8, 1988)

The two dimensional electron gas concentration dependence of photoreflectance of n-A1GaAs/GaAs heterostructures are reported and explained by analysing the physical process induced by photo-modulation. Comparison of the experimental results and energy level calculations based on a triangular potential well approximation shows good agreement.

With the development of molecular beam epitaxy

concentration of 1-2xl0mcm+ and a 100A-thick top

(MBE), it becomes possible to grow selectively doped

cladding GaAs layer were grown. The mole fraction of

n-A1GaAs/GaAs heterostructures with a high quality

AlAs in A1GaAs is about 0.3 (as grown). The sample structure is shown in Fig.1. Hall measurements at 77K

two-dimensional electron gas (2DEG). Due to various possible applications of this kind of structure, a lot of work has been reported concerning them, with the main emphasis on transport propertiesJ such as the quantum Hall effect, z3

show that the 2DEG concentrations of the samples are 0.5-1.2xl0tZcm-2 with electron mobility of 0.9-1.3xl0,cm2/Vs.

Photoreflectance (PR) is a powerful tool in the studies of semiconductor microstructures, but very few PR studies of the 2DEG system in n-A1GaAs/GaAs heterostructures have been reported. +.5 In this paper, PR spectra of n-A1GaAs/GaAs single heterojunctions with different 2DEG concentrations are reported. A simple model is introduced to explain the experimental results. The samples were grown by molecular beam epitaxy. Firstly, a 2-3 #m-thick undoped GaAs layer was grown on a (001) Cr-doped GaAs substrate. Then a 100A -thick spacer A1GaAs layer was introduced for reducing impurity scattering and improving the interface quality. Finally, a 600A-thick

n(Si)-doped

A1GaAs layer

0749-6036/89/080391 +03 $02.00/0

with

doping

The experimental arrangement is similar to that reported by Shay.+ The pump beam is the 6328A line of a He-Ne laser chopped at 125Hz. For improving the signal-to-noise ratio, a silicon PIN diode was used as the detector. Shown in Fig.2 are the room temperature PR spectra of the samples. The features at 1.420eV are assigned to be due to the direct bandgap in the GaAs buffer layer. With the increase of the 2DEG concentration, the second feature in each spectrum shifts to high energy. These features are believed to be related to the 2DEG at the heterointerface rather than to the A1GaAs layer. PR measurements also show that the intensity of the © 1989 Academic Press Limited

Superlattices and Microstructures, Vol. 6, No. 4, 1989

392

band states in the potential well region are still similar to

N-GaA[As spacer

those of a 3D crystal, it will not influence the formation of quasi-2D critical points in the 2DEG region. When incident light with a polarisation component perpendicular to the

undoped-OclAs

material growth direction (Z direction) illuminates the sample, it should be possible to observe the 2DEG-related features in PR measurements. Based on the triangular well

SUB.

approximation s and Hall measurements, the critical point energies in the 2DEG region can be obtained as follows,

Fig.1

The schematic structure of the selectively doped n-A1GaAs/GaAs heterojunction samples.

E mEo =EcO~-EvC~,+E * =E go'*'+(2m*)-t~(]%2NJ~ )~rl

(1)

(i=0,1,2,...)

where Eg c'~ is the bandgap of GaAs; m* is the effective mass of electrons; "ti---qi/(2rc)is Planck constant; Nn, is the 2DEG concentration and r0=2.238, r1=4.087, r2=5.520 ....

m c3

For large i, we have

>-r~=[1.51t(i+3/4)] ze t./3 Z ,t.kJ b-Z

(2)

It is believed that the photo-modulation of a single heterojunction structure is mainly due to the modulation of the photo-excited carriers, which can change the 2DEG concentration so that the triangular well steepens or flattens and change the quantised sublevel energies as well. In the weak modulation case, by neglecting the modulation of the

1./-,,0

1.~.5

1..50

150

E (ev) Fig.2

broadening

parameters,

PR

of

n-A1GaAs/GaAs

heterostructures is mainly due to the modulation of the

The PR spectra of five samples measured at room

quantised sublevel energies, i.e. the critical point energies.

temperature. The 2DEG arrows indicate calculated

It produces the first derivative functional line shape and can

critical point energies.

be well fitted by ~,9 ,~R/R=Re {~Cnei~(E-Eo~°+iF ) " }

2DEG related signals and the spectral resolution depend strongly on the carrier mobility of the 2DEG and the interface quality of the sample. Hence PR can be used as a sensitive tool in determining the interface quality of the samples.

where C is the amplitude, 0 is the phase angle, E m~Gis the tcansition energy and F is the broadening parameter. The summation is over all critical points. For 2D system without excitonic effects, we have m=l. The least square fit of the spectra to eq.(3) can give the

As is well known, the features in PR spectra correspond to the critical points (van Hove singularities) in the band structures

(3)

of

the

materials. 7 In

a

n-A1GaAs/GaAs

heterostructure, there exists a triangular potential well in the conduction band of GaAs near the heterointerface. The

critical energies and the broadening parameters. The experimental and calculated results are all listed in Table 1. It can be seen that the critical point energy increases with the increase of the 2DEG concentration. The experimental results agree with the theoretical calculations quite well.

electrons are distributed on the quantised subbands in the well region to form a 2D system. Although the valence

When the interface quality of a sample is not good, the

Superlattices and Microstructures, Vol. 6, No. 4, 1989 Table 1. The experimental and calculated data of the samples ,N°

N~

critical

point

broadening

energy

(eV)

parameters

exp.

theo.

(meV)

1 5.6x10 n 1.493

1.494

2 3 4 5

1.502 1.506 1.507 1.527

12.5 10.3 14.1 11.0 11.7

(cm"2)

6.7x1011 7.0x10 n 7.5x10 ~1 1.1xl012

1.502 1.507 1.520 1.528

impurity scattering will reduce the carrier mobility of the 2DEG, which may broaden the spectral features and decrease the spectral resolution or even lead to the disappearance of any PR features related to 2DEG. This is just what we observed in PR measurements of some samples. In conclusion, we have measured the 2DEG concentration dependent PR of n-A1GaAs/GaAs heterojunctions. Based on the triangular potential well model, the critical point energies in the 2DEG region are calculated. The experimental results obtained by fitting the

393

spectra to the first derivative functional line shape agree well with the theoretical calculations. Acknowledgements-The authors acknowledge the financial support of the National Science foundation of China (NSFC) and the Structure Analysis Lab, University of Science and Technology of China. References 1. See T.Ando, A.B.Fowler and F.Stern, Rev. Mod. Phys. 54, 437(1982) 2. K.von Klitzing, Rev. Mod. Phys. 58, 519(1986) 3. D.C.Tsui and H.C.Stroemmer, IEEE J. Quantum Electronics QE-22, 1711(1986) 4. O.J.Glembocki, B.V.Shanabrook, N.Bottka, W.T.Beard and J.Comas, Appl. Phys. Lett. 46, 970(1985) 5. Y.S.Tang and D.S.Jiang, Chin. Phys. Lett. 4, 283(1987) 6. J.L.Shay, Phys. Rev. B2, 803(1970) 7. Y.Hamakawa and T.Nishino, Optical Properties of Solids--New Developments, ed. by B.O.Seraphin (North Holland, N.Y., 1976), 256 8. Y.S.Tang, D.S.Jiang and K.Ploog, Solid State Commun. 64, 655(1987) 9. R.Enderlein, D.S.Jiang and Y.S.Tang, Phys. St. Sol. (b) 145, 167(1988)