Intersubband absorption in Sb doped Si/Si1x Ge x quantum well structuresgrown on Si (110)Chanho Lee and K. L. Wang

Citation: Applied Physics Letters 60, 2264 (1992); doi: 10.1063/1.107049 View online: http://dx.doi.org/10.1063/1.107049 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/60/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Intersubband relaxation time in the valence band of Si/Si1x Ge x quantum wells Appl. Phys. Lett. 69, 3069 (1996); 10.1063/1.116842 Intersubband absorption in Si/Si1xy Ge x C y quantum wells Appl. Phys. Lett. 69, 1734 (1996); 10.1063/1.118013 Electron intersubband absorption in Ge/Si1x Ge x quantumwell structures grown on Si (001) substrate Appl. Phys. Lett. 64, 1256 (1994); 10.1063/1.110857 Boundtobound intersubband transitions in a doped ptype Si/Si x Ge1x /Si quantum well Appl. Phys. Lett. 62, 1119 (1993); 10.1063/1.108761 Intersubband absorption in the conduction band of Si/Si1x Ge x multiple quantum wells Appl. Phys. Lett. 59, 2977 (1991); 10.1063/1.105817

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lntersubband absorption in Sb S-doped Si/Si,-,Ge, quantum well structures grown on Si (110)

Chanho Lee and K. L. Wang Device Research Laboratory, Departmen f of Electrical Engineering, University of California, Los Angeles, Los Angeles, California 90024-1594

(Received 3 December 1991; accepted for publication 2 March 1992)

Strong electron intersubband infrared absorption is observed for Sb S-doped Si/Sii-,Ge, multiple quantum well structures grown on ( 110) Si substrates. The intersubband absorption is shown to be allowed for both the optical field components perpendicular and parallel to the quantum wells due to the tilted ellipsoidal of constant energy surfaces. About 90% infrared absorption is measured by a Fourier transform infrared spectrometer using a waveguide structure with 10 internal reflections. For various samples used in experiments, absorption peaks ranging from 4.9 to 5.8 pm are observed. The peak energy is shown to be tunable by changing the Ge composition in the Sii-,Ge, barriers and the doping concentration in the Si quantum wells.

Intersubband absorption in quantum well structures has been of great interest due to its potential application for infrared (IR) detectors. The first intersubband absorption has been observed in GaAs/AlGaAs multiple quantum wells.’ Recent advances in silicon molecular beam epitaxy (MBE) technology have made it possible to observe the intersubband absorption in Si-based multiple quantum well structures. For example, hole and electron intersubband absorptions have been observed in Si/Sit -,GeX multiple quantum wells grown on (001) Si substrates.2’3 The high doping capability in Si results in high absorption strength comparable to or higher than those observed in multiple quantum wells in GaAs/AlGaAs heterostructures.“4 The possibility of monolithic integration with electronic devices using Si technology makes the use of Si or SiGe/Si multiple quantum wells more attractive.

However, for all these structures, there is a drawback due to the selection rule which limits the detection or ab- sorption when light is incident normal to the quantum well layers. On the other hand, it has been recognized, in an earlier investigation of far-infrared spectroscopy of inver- sion layers in metal-oxide-semiconductor (MOS) struc- tures,’ that normal incident light can induce the electron intersubband transitions in Si ( 110) due to the tilted ellip- soidal. There was a report of the detection of the derivative of absorption due to the electron intersubband absorption with an optical field parallel to the inversion layer of ( 110) Si MOS.6 In the previous case, the intersubband absorption occurs in the single inversion layer, and the absorption strength is usually very small for any practical application. More significantly, the transition energy is usually in the far-IR range due to the heavy effective mass of the con- duction band electrons.

In this letter, we report the first observation of the electron intersubband absorption in Sit -,Ge,/Si quantum wells grown on ( 110) Si substrates with the optical field parallel to the layers, .i.e., detection of incident light normal to the plane of the quantum wells. The peak energy posi- tions of absorption spectra are shown to vary with the Ge compositions as well as the doping concentrations in the quantum wells. The transition energy is tunable from far-

IR to a few ,um in wavelength. The polarization angle dependencies of the absorption spectra are obtained and compared with those observed for samples grown on (001) Si substrates.

Samples used for this study are grown in a Si-MBE system and n-type doping is achieved by Sb thermal evap- oration at a low temperature.7’8 Details of the growth are described elsewhere.’ The sample structure is shown in Fig. 1 (a). Prior to the growth of the quantum wells, a Sit-,,Ge, buffer layer of l-l.5 pm thick is grown first, and the Ge composition x of the barrier is varied from 0 to 50% in the present experiment. The Ge content (v) in the buffer layer is chosen so as to maintain the symmetric strain condition in the multiple quantum wells.” More im- portantly, this is done in order to provide a favorable con- dition for normal incidence detection as will be discussed later. Si well layers are grown below 400 “C! for the desired sharp and high doping profile. The doping concentration of the Si layers is about 1.3X lo*’ cm -3 according to second- ary ion mass spectrometry (SIMS) profiles. Three samples are prepared and they have the same doping concentration in the quantum wells but with different Ge compositions in the barriers as shown in the inset of Fig. 2. Details of the measurement procedure are described elsewhere.’ For (110) substrate, two waveguides with wedges along [ilO] and [OOl] directions are prepared for each sample and these directions are illustrated in Fig. 1 (b).

Figure 2 shows transition energy, normalized absorp- tion strength per quantum well, and full width at half max- imum (FWHM) of three samples with different Ge com- positions in barrier layers when the 0” polarization angle is used. The polarization angle 8 is defined as shown in the inset of Fig. 3, where the 0” polarization angle is shown and the 90” polarization angle represents an optical field of the incident light along the quantum well plane and waveguide wedges. The absorption peak position (transition energy) of sample A is 216 meV (5.8 pm) and those of samples B and C are 226 meV (5.5 pm) and 253 meV (4.9 pm), respectively. The transition energy increases as the Ge composition increases. This is due to the fact that the con- duction band offset of Si/SiGe heterostructures for the X4

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SiGe Cap Layer (2500A)

( 10 Periods )

SiGe Buffer Layer (1-l.Spm)

Undoped SiGe (300A)

Si Substrata (110)

(4

[OOI] Direction

A

FIG. 1. (a) Sample structure. The structure consists of a l-l.5 pm un- doped Si, -,,Ge,/Si buffer layer followed by 10 periods of a 50 b; Si layer doped pith Sb in the 40 %, center rt$on, an undoped Si, -,Ge/Si barrier of 300 A thick, and finally a 2500 A undoped Si,-,Ge,JSi cap layer. (b) Directions of sample wedges. Two waveguide structures are prepared for each sample so that the incident optical field can be selected along [ilO] and [OOl] direction, respectively.

valleys becomes large as the Ge composit ion increases. The potential wells become deeper due to the combined effects of increasing conduction band offset and S doping, result- ing in larger energy level separation between subbands. W ith S doping as in our case, higher transition energies can be obtained due to the HartreeFock potential and many body effects as compared with simple SiGe/Si and MOS inversion quantum well cases. It is also observed that the absorption strength decreases as the Ge composit ion in-

260 1.1 a,

220

7 E” 2180

Ge Composit ion [“T&I

FIG. 2. Ge composit ion dependence of transition energy, normalized absorption strength per quantum well, and F W H M for the 0” polariza- tion. The Ge composit ions of samples A, B, and C are 0, 30%. and SO%, respectively. The peak energy position (transition energy) shifts to higher energies as the Ge composit ion is increased. The structure description for the three samples is summarized in the inset.

1

0.8

G 5 0.6 2 8 g! 0.4

4, 6 d

3:

%i 2g

$ 0

1 .E E.

B n a

0 100 200 300 Energy [meV]

400 5OC.Y

FIG. 3. Polarization angle dependence of absorption spectra of sample B. The polarization angle 0 is defined in the inset. The absorption spectrum is observed at 8=90” due to intersubband transition excited by the parallel field. The peak energy position shifts and the increase of F W H M is also observed as the polarization angle is increased.

creases. The full widths at half maximum of absorption spectra are almost the same with different Ge composi- tions.

The polarization angle dependence of absorption spec- tra of sample B is shown in Fig. 3. Other samples show similar dependencies. The absorption strength is at the maximum for the 0” polarization angle. The peak value in absorbance corresponds to an absorption of 90% in the sample. At large polarization angles, the absorption arises from the intersubband transition of electrons in the X4 valleys excited by the parallel field. The “parallel field” is defined as the optical field component along the quantum well planes and the waveguide wedges [i.e., [ilO] direction in Fig. l(b)], and the “perpendicular field” is defined as the optical field along the quantum well growth direction [i.e., [l lo] direction in Fig. 1 (b)]. The absorption peak position at the 0” polarization angle is 226 meV (5.5 pm) and 177 meV (7.0 ,um) at 90“, respectively. The reason why the peak position shifts to lower energies at large po- larization angles may be in part due to the depolarization effect.6 It is also observed that the absorption spectra be- come broader at larger polarization angles. The FWHM of the spectrum at the 0” polarization is 166 meV and 189 meV for the 90” case, respectively. The polarization angle dependence of absorption strength of sample B for the op- tical field along both the [ilO] and [OOl] wedge directions are summarized with open circles and squares in Fig. 4. The plotted absorption strength represents the integrated area under the absorption spectrum.

Next, we discuss the condition to have normal inci- dence detection. When the polarization angle is go”, the intersubband absorption is induced by the parallel field (i.e., normal incident light j. The parallel field cannot cause any intersubband transitions for (001) substrates. For (110) substrates, the mass tensor of electrons in the X4 valleys (along [loo], [iOO], [OlO], [OiO] directions) has both diagonal and off-diagonal terms due to the four tilted energy ellipsoidals as shown in Fig. 1 (b). The diagonal terms represent the electron motion along the growth di-

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_ 1.21, g 8 I c ( 8 I i 3 8 II L 11 t r a I ’ 1 A Liz 1 Fi ‘%Q g 0.8 \ 0 0

\ k \fJ g 0.6

s R

0.4

5 0.2 .- E. 0 [OOI] Experimental . $ 0

2 -0.2 -20 0 20 40 60 80 100

Polarization Angle 0

FIG. 4. Absorption strength vs polarization angle for sample B. For the optical field along the [llO] direction, the absorption strength decreases first and increases again as the polarization angle is increased. For the optical field along the [OOl] direction, the absorption strength decreases in accordance with the cos’ tl rule.

rection induced by the perpendicular field while the off- diagonal terms represent the electron motion along the quantum well growth direction by the parallel field. The projected effective mass of electrons in the four X4 ellip- soidal to the quantization direction (growth direction) is 0.32 rn@ Similarly the projected effective mass to a quan- tum well plane along the [ilO] direction is evaluated to be 0.47 mm The mass tensor of electrons in the X2 valleys (along [OOl] and [OOi] directions) has only diagonal terms and thus the electrons in the X2 valleys cannot give rise to the motion in the quantum well growth direction by the parallel field. Therefore, only the perpendicular field can induce the intersubband transition for electrons in the X2 valleys as in the case of (001) Si. The effective mass of electrons in the two X2 ellipsoidals is the transverse mass or 0.19 rn*

Thus, it is important that the X4 valleys should be occupied in order to have intersubband’ transitions excited by the parallel field, or in other words, to have normal incidence detection. For. the ( 110) SiGe/Si multiple quan- tum well structures shown in Fig. 1 (a), it is desired to have a proper strain to satisfy the condition. With the Si,-,Ge,, buffer layer, the Si layers suffer a tensile strain and the X2 and X4 energy bands in Si layers split, making the X4 valleys lower than the X2 valleys. The energy level sepa- ration of the X2 and X4 valleys are 85 meV for the S&Ge&Si multiple quantum well structures if the dop- ing effect is not considered. With this energy level separa- tion, most of the electrons will occupy the X4 valleys and this can explain the large absorption strength obtained at 8=90”.

To confirm the experiment further, we have calculated the absorption for the waveguide structure used in experi- ments. The calculated results are also shown in Fig. 4 for the [ilO] direction (solid line) and the [OOl] direction (dashed line, i.e., cos* 9), respectively. In the calculation, the projection of the incident light to the xy plane is as- sumed to be along the [ 1 lo] growth direction. It is assumed

that carriers occupy only the X4 valleys, and the intersub- band absorption can be evaluated similarly for all optical field polarizations with the use of different effective masses. Details of the calculation can be found elsewhere.” The transition energy has also been calculated using the Hartree-Fock potential, including the depolarization and the excitonlike eiTects.12 The transition energy obtained is 203 meV for sample B, compared to the value of X5 meV with the Hartree-Fock potential only. This value is close agreement with the experimentally observed value of 226 meV.

As shown in Fig. 4, the agreement is reasonable, but there is discrepancy. In the [OOl] direction case, the devi- ation at high polarization angles may be due to the diffi- culty in cutting the wedge at the exact [OOl] angle in ex- periments. In that case, the incident light is off the direction and the mass tensor has small off-diagonal terms. In the [ilO] direction case, the absorption comes from the parallel and perpendicular fields and the attribute of the discrepancy is more difficult to assess in this case. It may be due to different effective masses and the depolarization ef- fect. More work needs to be done in this area.

In summary, we observed the normal incident electron intersubband absorption on the (110) SiGe/Si quantum wells under a proper strain condition. The absorption of 90% at the 0” polarization angle and the absorption of 88% for 90”, respectively, are obtained with a waveguide struc- ture of ten multiple quantum wells. A significant portion of the quantum well energy is contributed by the S-doped Hartree-Fock potential (exchange energy) and other many body effects in addition to the band offset of hetero- structures. It is shown by changing both Ge composition and doping concentration in the well layers that the tran- sition energy is easily tunable to cover a larger energy range in spite of the relatively large effective mass. The result offers the opportunity for the application of infrared focal plane detectors and modulators.

I would like to thank R. P. G. Karunasiri for helpful discussions. This work was in part supported by AR0 (Dr. John Zavada) and AFOSR (G. Witt) .

‘L. C West and S. J. Eglash, Appl. Phys. Lett. 46, 1156 (1985). *R P. G. Karunasiri, J. S. Park, Y. J. Mii, and K. L. Wang, Appl. Phys.

Lett. 57, 2585 (1990). s H. Her& G. Schuberth, E. Gornik, and G. Absreiter, Mater. Res. Sot.

Symp. Proc. 220, 379 (1991). 4B. F. Levine, K. K. Choi, C. G. Bethea, 3. Walker, and R. J. Malik,

Appl. Phys. Lett. 50, 1092 (1987). sT. Ando, 2. Phys. B 26, 263 (1977). 6S. M. Nee, U. Claessen, and F. Koch, Phys. Rev. B 29, 3449 (1984). 7H.-J. Gossmann, E. F. Schubert, D. J. Eaglesham, and M. Cerullo,

Appl. Phys. Lett. 57, 2440 (1990). sS. S. Iyer, K. A. Metzger, and F. G. Allen, J. Appl. Phys. 52, 5608

(1981). ‘C. Lee and K. L. Wang, J. Vat. Sci. Technol. B 10, 992 ( 1992).

“E. Kasper, H.-J. Herzog, H. Jorke, and G. Abstreiter, Superlatt. Mi- crostruct. 3, 141 (1987).

“S. K. Chun and K. L. Wang (unpublished). ‘*R. P. G. Karunasiri, J. S. Park, K. L. Wang, and S. S. Rhee (unpub-

lished) .

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