Thermoreflectance spectra of Cd3As2

1 Phys Chom Sohds,1977. Vol 38. pp 1237-1238

Permmon Press Pnnted I” Great Brawn

SPECTRA OF Cd,As,

THERMOREFLECTANCE Department

V. P. BHOLA of Physics, Umverstty of Sherbrooke, Sherbrooke, Quebec, Canada (Received 29 November 1976; accepted 25 February 1977)

Ah&act-The near normal incidence thermoreflectance spectra of Cd,As, have been investigated at room temperature and hqmd mtrogen temperature m the spectral range 2.5-6.2 eV.

1. lNTROIWCl’lON

The thermoreflectance technique has been used by many research workers [ l-41 to investigate the energy bands of semiconductors. In this paper we present the thermoreflectance spectra of CdsAsz at room temperature and liquid nitrogen temperature in the energy range 2.8-6.2eV. Transitions observed are identified on the basis of the reflection measurements of Sobolev et al. [S], and the theoretical calculated values of Lin-Chung[6]. CdJAsZ is a degenerate n-type semiconductor with a narrow gap, small electron effective mass and high mobility. Sobolev et al.[5] have investigated the reflection spectra of CdsAsl and Zn,Asz to 12eV. Zivitz and Stevenson[7] have extended this region upto 21 eV. Band structure calculations were made available by LinChung[6] and she has obtained numerical results for many points and lines in the Brillouin zone. Aubin and Cloutier[4] have published the thermoreflectance spectra of Cds-,Zn,Asz. They have also presented the results of Cd,Asz for photon energies 1.5< Ec3.5. In contrast, they have identified the transitions El, El + A, at 1.8 and 2.01 eV respectively. They have also observed transitions X&X, at 3.24 eV.

entrance slit of 0.5 m scanning Ebert spectrometer model No. 82OOfl.Light from the exit slit of spectrometer was focussed normally on the etched surface of the sample. The light beam reflected from the sample was refocussed on a RCA photomultiplier tube lP28. The a.c. portion of the signal aAR, from the output of photomultiplier was fed to the lock-in amplifier, which was tuned to the modulation frequency. The output from the lock-in amplilier was recorded on a strip chart recorder. The d.c. portion aR was recorded separately. AR/R was obtained for a number of points. Fiie 1 shows the plot of AR/R vs the photon energy. 3. RMIJLSANDLMscussIoN Thermoreflectance spectra of Cd,Asz were measured on four samples. A typical spectrum is presented in Fig. 1. In each measurement the results were quite reproducible and the spectral structure of the peaks was the same. However, the absolute value of AR/R diiered from sample to sample. The optical resolution of the data is 16 A which gives an energy resolution of O.Oll0.043 eV. It has been pointed out by Lin-Chung[6], that there exists a resemblance between the band structure of

The method of preparation for single crystals of Cd,Asl is described elsewhere@]. The back surface of the sample was left rough, while the front surface was polished mechanically. Samples were ground down to a thickness of 150-200 pm, so as to keep the heat capacity at a minimum. The sample was then etched in concentrated HCl for 40-60 sect The sample was bonded with vacuum grease to the germanium heater, which in turn was fixed on the cold finger of a metal cryostat fitted with quartz windows. The cryostat was evacuated to approximately 10m6torr; and coolants such as liquid nitrogen or ice + water were used. The heater was electrically insulated from the copper plate by using a thin layer of epoxy between the two. A unidirectional square wave voltage of frequency 11 Hz from a Wavetek model No. 110 was applied to the germanium heater. The same square wave generator was also used to supply the reference signal to PAR model No. 121 lock-in amplifier. Light from a IOOOWxenon lamp was focussed on the

4

I 3

tAfter etchmg the sample with HCI, tt took about IO-15min at RT and about 40-60 mm at LNT before measurements of AR and R were started JPCS Vol 38. No

II--B

E4 ,

, E5

4

5 Energy,

Es , 6

eV

Pii. 1. Thermoretfectance spectra of Cd,As,. (a) Liquid nitrogen temperature, scale on r.h.s. (b) Room temperature, scale a*- ’ ’

1237

1238

V. P.

Table I Values of thermoreflectance peaks expressed in eV Peaks

EZ Es

El Ex

E6

Bhola LNT RT

Lin-Chung Probable Sobolev location RT LNT

2.9 3.26 31 4.6 5.16 5.63

2.86 3 33 31 5.15 -

2.9 3.22 3.1 -

2.86 326 3.1 -

2.8 30 38 47 5.2 5.8

W, x;_x, As-A, AX-A, X&X, J,,-P;

III-V, i.e. InAs, GaAs, GaSb and AlSb etc. and II-V compounds. In Table 1, our results for the thermoreflectance spectra of Cd3As2 at RT and at LNT have been compared with her theoretical calculated values and with the observed values by Sobolev[S]. The transition Z&Z, at 2.9eV has been observed at room temperature and at liquid nitrogen temperature. The value is close to the value obtained by Sobolev[5] (see Table 1). It is labelled as E2. Theoretically the value of E2 has been predicted to be 2.8 eV [6]. According to Lin-Chung, the value of the energy gap X&X, is 3.0eV. She has compared this value with the experimental value of 3.65 eV (see Ref. [6]). As a matter of fact the two values at 3.0 and 3.65eV belong to two different transitions. Experimentally at RT and at LNT, we have observed the X;-X, transition at 3.26 and at 3.22eV, respectively. We have labelled this transition E,. At RT Aubin[4] has observed it at 3.24 eV. The peak E4 has been observed at 3.7eV. It is attributed to the &A, transition. Sobolev[5] has observed it at the same position, and Lin-Chung[6] has calculated its value as 3.8 eV. Recently Stevenson[7] has identified a peak at 3.6 eV as E3,,, but the value of 3.6 eV is much closer to the value of the peak E, than that of E3 (see Table 1).

BHOLA

From the thermoreflectance experiment on CdsAsl at RT, we have measured the energies of two components of Es, at 4.6 and 5.16eV respectively. Experimentally Sobolev[5] has observed only one component from the reflectivity measurements. The calculated values of the two components are 4.7 and 5.2 eV and our results are in agreement with the theoretical results. By comparing with Sobolev’s results, these transitions are identified as A,-& and X&X,. Stevenson has observed one component at 4.5 eV (see Ref. [6]). We have observed &, i e I,rG transition at 5.63 eV, while its theoretical value is 5.8 eV. At LNT we did not observe the transitions Es and EC due to a weak signal. We hope to improve the equipment and extend the region. 4. CONCLUSION By employing the thermoreflectance technique, we have observed the peaks Ez, E,, E,, Es and E6 as labelled by Sobolev[5]. The peaks are well resolved and the data are in agreement with the reflectivity measurements of Sobolev and theoretical values of Lin-Chung[6].

REFERENCES

1. Batz B., Semiconductors

and Semimetals, Vol 9 Academrc Press, New York (1972). 2. Cardona M , Solid State Phys. Suppl. II (1%9) 3. Matatagui E., Thompson A. G and Cardona M., Phys Rev. 176, 950 (1968).

4. Aubin M. J. and Cloutier J P , Can. 1. Phys. 53, 1642(1975) 5. Sobolev V. V., Syrbu N N., Zyubina T A and Ugai Y A., Sow. Phys. Semiconductors 5, 279 (1971). 6. Lit&hung P. J., Phys. Rev. 188, 1272(1%9) 7. Zivitz M. and Stevenson J R., Phys. Rev. E. 10, 2457 (3974) 8. Rambo A., M SC. Thesis, Umverstty of Sherbrooke, Sherbrooke, Quebec, Canada (1975)