D. P. Wang

Fast Fourier transform of photoreflectance spectroscopy of δ‐doped GaAs

The photoreflectance (PR) spectroscopy of δ‐doped or similar structured sample has been observed to exhibit many Franz–Keldysh oscillations (FKOs). The beats are shown in the FKOs and they are attributed to the different frequencies of the FKOs of the transitions from the heavy and light holes. We have applied the fast Fourier transform on the PR spectra to separate the contributions from the heavy and light holes. The electric fields in the undoped layer can be calculated from the frequencies of heavy and light holes, respectively. The result of the heavy hole agrees well with that from the conventional FKOs fittings if the reduced mass=0.055 m0 is used in the conventional fittings.

InN nanotips as excellent field emitters

Unidirectional single crystalline InN nanoemitters were fabricated on the silicon (111) substrate via ion etching. These InN nanoemitters showed excellent field emission properties with the threshold field as low as 0.9V∕μm based on the criterion of 1μA∕cm2 field emission current density. This superior property is ascribed to the double enhancement of (1) the geometrical factor of the InN nanostructures and (2) the inherently high carrier concentration of the degenerate InN semiconductor with surface electron accumulation layer induced downward band bending effect that significantly reduced the effective electron tunneling barrier even under very low external field.

Determination of built-in field by applying fast Fourier transform to the photoreflectance of surface-intrinsic n+-type doped GaAs

Photoreflectance spectroscopy of surface-intrinsic n+-doped (s-i-n+) GaAs has been measured at various power densities (Ppu) of a pump beam. Many Franz–Keldysh oscillations (FKOs) were observed above the band-gap energy, which will enable the electric-field strength (F) to be determined from the periods of the FKOs. Field F thus obtained is subject to photovoltaic effects. In order to reduce the photovoltaic effects from the pump beam, Ppu was kept below 10 μW/cm2 in the previous experiments. Here, we demonstrate that the built-in field can be determined at a larger Ppu by using fast Fourier transform techniques.

Contactless electroreflectance and photoreflectance studies of n- and p-type-doped GaN with Ga and N face

GaN films with Ga or N face and of n- or p-type doping were grown by plasma-assisted molecular-beam epitaxy. The wurtzite GaN exhibits a large polarization, with spontaneous and piezoelectric components. The direction of polarization, and thus polarization-induced electric fields, Fp is determined by the polarity of the sample. Contactless electroreflectance (CER) has already been employed to study the nature of band bending in GaAs samples. In this work, CER is used to study the band bending of the GaN samples. It was found that the phase of CER is determined by the type of doping, rather than by the polarity of the surface. Additionally, the photoreflectance (PR) spectra were also measured. Comparing CER with PR determines the type of doping.

Critical point energy as a function of electric field determined by electroreflectance of surface-intrinsic-n+ type doped GaAs

Electroreflectance of surface-intrinsic-n+ type doped GaAs has been measured over a various biased voltage. The spectra have exhibited many Franz–Keldysh oscillations (FKOs) above band gap energy Eg. The electric field F and critical point energy Ec can be determined from the slope and intercept of FKOs fitting. Hence, we can obtain Ec as a function of F. In most of previous works, Ec is taken as Eg. However, it was found that Ec increases with F in this work. In order to explain this, the gain of energy of electron and hole in F was discussed.

Effect of modulating field on photoreflectance simulated by electroreflectance

Photoreflectance (PR) of surface-intrinsic-n+ (s-i-n+) type doped GaAs has been simulated by electroreflectance (ER). The simulated spectra of the s-i-n+ sample have exhibited many Franz–Keldysh oscillations, which enable the electric field (F) to be determined. It is known that F’s determined from PR are subjected to photovoltaic effect and the measured F is close to Fbi−δF/2 when the modulating field, δF≪Fbi, where Fbi is the built-in field of the sample and δF is the modulating field. In this work, we have investigated the relation between the measured F and δF not only for the region where δF≪Fbi holds, but also for a whole range of δF. In order to determine the magnitude of δF, we have used ER to simulate PR, that is, the measurements of ER under a forward bias, which is set to be equal to δF/2.

Electroreflectance of surface-intrinsic- n(+)-type doped GaAs

The electroreflectance spectra of surface-intrinsic- n+-type-doped GaAs were measured at various bias voltages (Vbias). Results revealed many Franz–Keldysh oscillations (FKOs) above the band-gap energy, which have been attributed to a uniform electric field (F) in the undoped layer below the surface. However, there has been no other evidence for the uniformity of F in the undoped layer. Since it is known that F can be deduced from the periods of the FKOs, the relations between F and Vbias can, thereby, be obtained. The nearly linear relation, thus found, confirms the existence of a nearly uniform field in the undoped layer. From the plot of F against Vbias,the values of the thickness of the undoped layer and the barrier height can also be evaluated.

The effects of the magnitude of the modulation field on electroreflectance spectroscopy of undoped-n+ type doped GaAs

The electroreflectance (ER) spectra of an undoped-n+ type doped GaAs has been measured at various amplitudes of modulating fields (δF). Many Franz–Keldysh oscillations were observed above the band gap energy, thus enabling the electric field (F) in the undoped layer to be determined. The F is obtained by applying fast Fourier transformation to the ER spectra. When δF is small, the power spectrum can be clearly resolved into two peaks, which corresponds to heavy- and light-hole transitions. When δF is less than ∼1/8 of the built-in field (Fbi∼77 420 V/cm), the F deduced from the ER is almost independent of δF. However, when larger than this, F is increased with δF. Also, when δF is increased to larger than ∼1/8 of Fbi, a shoulder appears on the right side of the heavy-hole peak of the power spectrum. The separation between the main peak and the shoulder of the heavy-hole peak becomes wider as δF becomes larger.

Temperature dependence of Fermi level obtained by electroreflectance spectroscopy of undoped n+-type doped GaAs

The electroreflectance (ER) spectra of an undoped n+-type doped GaAs have been measured over a range of temperature from 25 to 400 K. Many Franz–Keldysh oscillations were observed above the band-gap energy, which enabled the electric field strength and, hence, also the Fermi level to be determined. The photovoltaic effect is shown to be negligible, even at the low temperature. The experiment shows that the Fermi level decreases with increasing temperature and has almost the same temperature dependence as the energy gap. It is pinned at about 0.63 of energy gap below the conduction band.

Determining of Infrared Transition of InN Film Grown on C-Plane Sapphire by Photoreflectance

An InN film was grown on sapphire (c-plane) by plasma-assisted molecular beam epitaxy, and its photoluminescence at 10 K and photoreflectance (PR) spectra from 10 K to 110 K were measured. Some prominent features in the PR spectra were observed in the infrared region below 120 K. The signals become too weak to observable for temperature above 110K. Furthermore, the binding energy of InN exciton was estimated to be 9.43 meV, which is equal to kBT at 109K. Therefore, the features in the PR spectra were assigned to the A, B, and C excitonic transitions associated with the direct gap of wurtzite InN. The thus obtained energies of the A, B, and C excitonic transitions versus temperature were fitted well by Varshini’s equation. The energies of the A, B, and C excitonic transitions at room temperature obtained by the best fit of Varshni’s equation are 0.738, 0.746, and 0.764 eV, respectively.