K. Wang

Mg doped InN and confirmation of free holes in InN

We report a systematic investigation on Mg doped InN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy. Electrolyte capacitance voltage (ECV) combined with thermopower measurements find p-type conduction over an Mg concentration range. For InN:Mg in this p-type “window” the Seebeck coefficients dramatically change their signs from negative to positive when the thickness of undoped InN interlayer decreases to zero. This notable sign change of Seebeck coefficient explains the previous inconsistency between ECV and thermopower results and confirms the existence of mobile holes in the InN:Mg. Taking into account the undoped InN interlayer, the hole density and mobility are extracted.

PN junction rectification in electrolyte gated Mg-doped InN

PN junction rectification is demonstrated in indium nitride. The junction is formed between the n-type surface accumulation layer and the Mg-doped, p-type bulk in a top-contacted thin film structure. The parasitic shorting path between the top contacts is controlled by gating with an electrolyte. For positive gate voltages, electrons accumulate at the surface and the current flows preferentially through n type regions, and linear current-voltage (I-V) behavior is observed. However, for negative gate voltages, surface electrons are depleted and current flows through the p-type bulk. This creates an n-p-n structure and a characteristic non-linear I-V curve is observed.

Effect of Mg doping on the structural and free-charge carrier properties of InN films

We study the structural and free-charge carrier properties of two sets of InN films grown by molecular beam epitaxy doped with different Mg concentrations from 1x1018 cm-3 to 3.9x1021 cm-3. We dete ...

Selective growth of N-polar InN through an in situ AlN mask on a sapphire substrate

Selective growth of N-polar InN by exploiting the ∼100° gap between the upper limits of the growth temperatures of In- and N- polarities has been introduced. An InN epilayer grown at a temperature in this gap on a sapphire substrate covered with an ultrathin AlN layer has demonstrated N-polarity. The dislocation density of such-grown InN layer has been significantly reduced (one order of magnitude lower than the conventional InN epilayers). The results have demonstrated great potential for improving the crystalline quality of hetero-epitaxial InN films. This concept can be easily adopted for other substrates.

P-type InGaN across the entire alloy composition range

A systematic investigation on Mg doped and undoped InGaN epilayers grown by plasma-assisted molecular beam epitaxy has been conducted. Single phase InGaN alloys across the entire composition range were synthesized and Mg was doped into InxGa1−xN (0.1 ≤ x ≤ 0.88) epilayers up to ∼1020/cm3. Hall effect, thermopower, and electrochemical capacitance voltage experimental results demonstrate the realization of p-type InGaN across the entire alloy composition range for properly Mg doped InGaN. Hole densities have been measured or estimated to be in the lower ∼1018/cm3 range when the net acceptor concentrations are in the lower ∼1019/cm3 range across the composition range.

Impact of Mg concentration on energy-band-depth profile of Mg-doped InN epilayers analyzed by hard X-ray photoelectron spectroscopy

The electronic structures of Mg-doped InN (Mg-InN) epilayers with the Mg concentration, [Mg], ranging from 1 × 1019 to 5 × 1019 cm−3 were systematically investigated by soft and hard X-ray photoelectron spectroscopies. The angle-resolved results on the core-level and valence band photoelectron spectra as a function of [Mg] revealed that the energy band of Mg-InN showed downward bending due to the n+ surface electron accumulation and p type layers formed in the bulk. With an increase in [Mg], the energy-band changed from monotonic to two-step n+p homojunction structures. The oxygen concentration rapidly increased at the middle-bulk region (∼4.5 to ∼7.5 nm) from the surface, which was one of the reasons of the transformation of two-step energy band.

Application of DERI method to InN/InGaN MQW, thick InGaN and InGaN/InGaN MQW structure growth

We proposed recently a new RF-MBE method called droplet elimination by radical-beam irradiation (DERI) for growing high-quality InN-based III-nitride films. DERI consists of two growth processes: a metal-rich growth process (MRGP) and a droplet elimination process (DEP). In InGaN growth, Ga was preferentially and selectively captured from the Ga/In wetting layer and droplets during MRGP. Then, excess In was swept out from the growing InGaN surface. The swept In was transformed to InN, epitaxially grown on top of InGaN during DEP using nitrogen radical beam irradiation. By repeating this process, an InN/InGaN MQW structure was successfully fabricated. Thick and uniform InGaN films were also successfully obtained by additionally irradiating the same Ga beam flux as that in MRGP even during DEP. When the irradiated Ga/N* beam flux ratio in DEP was changed from that in MRGP, an InxGa1-xN/InyGa1-yN MQW structure was successfully fabricated.

Temperature dependence of Mg-H local vibrational modes in heavily doped InN:Mg

We have studied the temperature dependence and anharmonic coupling of the local vibrational modes (LVMs) associated with Mg-H complexes in heavily doped InN:Mg. Two main LVM peaks are observed which are probably related to two different H-impurity bond lengths. The temperature dependence of the higher-frequency mode, which exhibits a monotonic frequency downshift and broadening with increasing temperature, can be explained by LVM dephasing due to acoustic phonon scattering. The lower-frequency mode displays an anomalous behavior as its frequency decreases initially and then starts to increase linearly above room temperature. The anharmonic coupling of the lower-frequency mode to a molecular mode of the impurity complex is suggested as a possible cause for this behavior.

New Low-Temperature Growth Method for High-Quality Low-Temperature GaN Layer by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

A new growth method has been proposed and verified to be effective for improving the crystalline quality of GaN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) at low temperature. In this new method, an indium (In) layer is predeposited before the main growth of low-temperature (LT) GaN. The improved quality for LT GaN probably comes from a selective growth process between In and gallium (Ga) in predeposited In coverage, which is similar to the process of several-monolayer-level liquid-phase epitaxy. This method may enable the extension of the application field of nitride semiconductors, for example, to solar cells.

In situ Investigation of Growth Mechanism during Molecular Beam Epitaxy of In-Polar InN

A real-time investigation of In-polar InN growth by radio-frequency plasma-assisted molecular beam epitaxy has been conducted using laser reflection and reflected high-energy electron diffraction (RHEED). Laser reflection intensity is very sensitive to surface changes in the growth front. Clear reflection intensity changes have been observed for In-stabilized surface, In adlayer, and In droplet formation. Simultaneously, the envelope of reflection intensity variation shows interference oscillation due to the increasing InN thickness. Laser reflection intensity increases during In deposition and decreases during the following N irradiation, in contrast to RHEED intensity. In situ monitoring using both techniques has demonstrated complementary information on the growth front. On the basis of the observation from both techniques, the growth mechanisms and roles of the In adlayer and droplets in the InN growth are discussed.