APPLIED PHYSICS LETTERS 98, 141105 ͑2011͒
S.-R. Jan,1 C.-Y. Chen,1 C.-H. Lee,1 S.-T. Chan,2 K.-L. Peng,2 C. W. Liu,1,2,3,a͒
Y. Yamamoto,4 and B. Tillack4,5
1Department of Electrical Engineering, Graduate Institute of Electronics Engineering,
National Taiwan University, Taipei 106, Taiwan
2Department of Electrical Engineering, Graduate Institute of Electro-optical Engineering,
National Taiwan University, Taipei 106, Taiwan
3National Nano Device Laboratory, Hsinchu 30078, Taiwan
4Innovations for High Performance Microelectronics, 25, 15236 Frankfurt (Oder), Germany
5Technische Universität Berlin, HFT4, Einsteinufer 25, 10587 Berlin, Germany
͑Received 2 December 2010; accepted 6 March 2011; published online 5 April 2011͒
The influences of defects and surface roughness on the indirect bandgap radiative transition of Ge
were studied. Bulk Ge has 15 times the integrated intensity of photoluminescence of Ge-on-Si.
However, for Ge-on-Si sample, the direct transition related photoluminescence intensity is higher
than the indirect transition related one. We affirm that the defects in the Ge-on-Si are responsible for
the weak indirect transition and relatively strong direct transition. The scattering of electrons by
roughness at Ge/oxide interface can provide extra momentum of the indirect band transition of Ge,
and thus enhance the indirect radiative transition. © 2011 American Institute of Physics.
The optical properties of Ge are of great interest due to
the high carrier mobility, strong photon absorption, and pos-
sible integration with Si. The direct conduction valley of Ge
is only 140 meV above the indirect valleys, and the enhance-
ment of direct transition is possible by increasing the elec-
tron population in the direct valley. The approaches to en-
increasing temperature, and tensile strain in Ge.3,4 Moreover,
pn junction was also observed.5 In this letter, the influence of
defects on indirect radiative transition of Ge is studied by
comparing bulk Ge and Ge grown on Si samples. The inter-
face roughness scattering is also studied by comparing a Ge
metal-insulator-semiconductor ͑MIS͒ light-emitting-diode
͑LED͒ and an n+p junction LED.
The Ge-on-Si sample is fabricated by reduced pressure
chemical vapor deposition system. A two-step deposition
was processed. At the first step, a Ge seed layer was grown
on Si at 300 °C in N2–GeH4 mixed ambient to form two-
dimensional Ge seed layer. The relaxed Ge film was then
grown at 550 °C in H2–GeH4 mixed ambient. The 550 °C
growth temperature is used for accelerating the growth rate
of Ge film. In order to improve crystallinity of the Ge layer,
several cycles of annealing steps were introduced during the
Ge layer deposition process by interrupting the Ge layer
growth.6 The thickness of the relaxed Ge layer is ϳ2.8 m
with estimated threading dislocation density of ϳ3.7
ϫ106 cm−2 observed by optical microscope after Secco
tively. The rapid thermal anneal was performed at 700 °C
for 60 s for dopant activation. The junction depth of n+ re-
gion is about 0.8 m measured by secondary ion mass spec-
trometry ͑SIMS͒.
For the MIS structure fabrication, a 50 °C liquid phase
deposition ͑LPD͒ was used to form ϳ2 nm thick gate oxide
on p-type Ge.7 Subsequently, a thin Al film ͑10 nm͒ was
evaporated on the SiO2 in a circular area with radius of
ϳ1.2 mm to form a gate electrode. After etching SiO2 on the
backside of the Ge wafer, 100 nm Al was evaporated on the
rear surface to form an ohmic contact. Moreover, 2 nm thick
Al2O3 is also used as gate dielectric to form another MIS
structure on an n-type Ge substrate by atomic layer deposi-
tion. The Al was evaporated as a gate electrode.
The room-temperature photoluminescence ͑PL͒ spectra
of the bulk n-type Ge substrate and Ge-on-Si sample are
shown in Fig. 2. The laser excitation wavelength is 671 nm
and the power density is 360 mW/mm2. The PL peaks at
695 meV and 780 meV are attributed to the indirect band
transition and direct band transition, respectively. The spec-
tra of the indirect and direct band gap emission in both bulk
n-Ge and Ge-on-Si sample were fitted by using the electron-
A ͑100͒ p-type Ge wafer with the thickness of 500 m
is used to fabricate the n+p junction device. The Ga-doped
Ge substrate has the resistivity of 1–10 ⍀ cm and the emit-
ter was formed by implanted phosphorous. The implant dose
and implant energy were 1ϫ1015 cm−2 and 60 keV, respec-
FIG. 1. ͑Color online͒ The optical microscope image of Ge-on-Si after
Secco etching. The threading dislocation density is ϳ3.7ϫ106 cm−2. The
defects are marked with circles. The inset is the TEM image of Ge-on-Si.
a͒
Author to whom correspondence should be addressed. Electronic mail:
0003-6951/2011/98͑14͒/141105/3/$30.00
98, 141105-1
© 2011 American Institute of Physics
129.174.21.5 On: Mon, 22 Dec 2014 22:34:38