Band-modulation of MgZnO / ZnO Metal-semiconductor-metal Photodetectors

Magnesium (Mg) diffusion behavior on the band modulation of MgxZn1-xO/ZnO metal-semiconductor-metal photodetectors (MSM-PDs) was studied. As the annealing temperature increases, Mg atoms diffuse from MgxZn1-xO into the underlying ZnO layer, which modulates the detection band of the fabricated MSM-PDs from two distinct bands into one band. For the annealing temperature lower than 900 oC, two detection bands were achieved located in the wavelength region of 280–320 nm and 360–400 nm, attributed to the absorption of the MgxZn1-xO and the ZnO layer, respectively. When the annealing temperature is raised to 900 oC, the MgxZn1-xO/ZnO bilayer becomes homogenized into a single MgxZn1-xO layer, leading to only one detection band with a wavelength region of 280–340 nm. In the photoluminescence measurement, the as-deposited MgxZn1-xO/ZnO bi-layer demonstrates two distinct emission peaks located at about 340 and 400 nm for the absorption of the MgxZn1-xO and ZnO layers, whereas only one emission peak of 355 nm was observed in the 900 oC-annealed MgxZn1-xO/ZnO bi-layer.


Introduction
ZnO and Mg x Zn 1-x O-based ultraviolet (UV) photodetectors (PDs) have been continuously studied because the ZnO material has many advantages, including a wide band gap (3.37 eV), high transparency (>80%) in the visible wavelength region, high exciton binding energy (60 meV), and non-toxicity [1,8].By mixing ZnO with another wide-direct-bandgap material, MgO (7.8 eV), the tunable bandgap material of Mg x Zn 1-x O, which can modulate the detection wavelength of PDs by varying Mg content, can be formed [9][10][11].In addition, no significant lattice distortion is found in the Mg x Zn 1-x O material because Mg +2 has a very similar ionic radius to that of Zn +2 [12].
Although GaN and Al x Ga 1-x N materials have been employed in various PDs, expensive and high-temperature technology was required to grow the GaN and Al x Ga 1-x N materials, including molecular beam epitaxy and metal organic chemical vapor deposition systems.In contrast, ZnO and Mg x Zn 1-x O materials can be grown using low-cost and low-temperature techniques, including radio-frequency (RF) magnetron sputtering and hydrothermal methods [13,14].As opposed to pure ZnO or Mg x Zn 1-x O materials, the Mg x Zn 1-x O/ZnO bi-layer heterostructure is a promising technological platform as evidenced heterojunction field-effect transistor, multi-quantum-well (MQW) light-emitting diodes, MQW-PDs, and optoelectronic devices with superlattice, as well as two-dimensional electron gas (2DEG) structures [15].
Previously, many Mg x Zn 1-x O/ZnO heterojunction UV-PDs were fabricated .As compared to the MSM-PDs without the Mg x Zn 1-x O capping layer, the PDs with the Mg x Zn 1-x O/ZnO bi-layer presented higher dark current, photocurrent, and photoresponsivity due to the shielding of ambient oxygen, defect, and surface states passivation by the Mg x Zn 1-x O capping layer [21].The Au/Mg x Zn 1-x O/ZnO PDs had larger responsivity than Au/ZnO/Mg x Zn 1-x O PDs [22].By varying the applied bias voltage, the detection wavelength of the Mg x Zn 1-x O/ZnO MSM-PDs could be modulated from a single to a dual wavelength [23].MgZnO/ZnO bi-layer with 2DEG behavior was investigated [24][25][26].
The photoluminescence (PL) was investigated by annealing the Mg x Zn 1-x O/ZnO bilayer, which tuned the Mg composition [27].Dual-band Mg x Zn 1-x O UV-PDs were fabricated by employing two Mg x Zn 1-x O layers with different Mg compositions [28].The Mg atomic reconstruction was observed in p-type interface (ZnO on Zn-polar MgZnO), but not in n-type interface (MgZnO on Zn-polar ZnO) due to the different polarity of the interface [29].The Mg atomic reconstruction was not caused by thermal diffusion, instead by the asymmetry of energy scales.
In this work, the Mg thermal diffusion behavior on the MSM-PDs with the Mg x Zn 1x O/ZnO bi-layer was studied.We found that by increasing the annealing temperature of the Mg x Zn 1-x O/ZnO bi-layer, one could modulate the detection band of the fabricated MSM-PDs from two bands into one band.

Experiments
ZnO and Mg x Zn 1-x O layers with a thickness of 250 nm were deposited consecutively on a sapphire substrate using an RF magnetron sputtering system with a substrate temperature of 200 ºC in a 10-mTorr Ar atmosphere.X-ray photoelectron spectroscopy showed the Mg content being 0.3 in the as-deposited Mg x Zn 1-x O film.Then, the Mg x Zn 1-x O/ZnO bi-layers were annealed at various temperatures between 700 and 900 ºC for 2 h, which forced the Mg atom to diffuse from Mg x Zn 1-x O to the underlying ZnO layer.The as-deposited (not annealed) Mg x Zn 1-x O/ZnO bi-layer was also prepared for comparison.MSM-PDs were fabricated by evaporating Au electrodes on the Mg x Zn 1-x O surface in an interdigitated pattern.Inter-diffusion behavior of Mg atoms was studied by absorption, PL, and secondary ion mass spectrometry (SIMS) measurements.Current-voltage (I-V) characteristics were recorded using a Keithley 2400 source meter, and the photoresponse was measured with a monochromator by illuminating the samples from the Mg x Zn 1-x O side with a 300-W Xe arc lamp.

Results and discussions
Dark I-V characteristics of the fabricated MSM-PDs for the as-deposited and varioustemperature-annealed Mg x Zn 1-x O/ZnO bi-layer are shown in Fig. 1.Clearly, the current increases with annealed temperature because the sheet resistance of the Mg x Zn 1-x O and ZnO layers was reduced by the thermal energy of the annealing process.With increasing annealing temperature, the sheet resistance of the ZnO and Mg x Zn 1-x O layers decreased from 179 MΩ/□ and 189 MΩ/□ for the as-deposited films to 0.6 KΩ/□ and 100 KΩ/□ for the ones annealed at 900 ºC.In the MSM-PDs with as-deposited and 700-800ºC annealing, the sharp increasing band of region I, wavelength region of 360-400 nm, originated from the absorption of ZnO layer.However, the sharp increasing band of region II comes from the absorption of the Mg x Zn 1x O layer, having a wavelength region of 280-320 nm.In contrast, the absorption band of ZnO disappears and only one absorption band is observed in the MSM-PDs with 900 ºC annealed Mg x Zn 1-x O/ZnO bi-layer.This absorption band has a wavelength region of 280-340 nm, which is shorter than the absorption region (360-400 nm) of ZnO.Thus, the one absorption band (280-340 nm) is an absorption result of the Mg x Zn 1-x O layer with Mg content (x value) less than 0.3 for the as-deposited Mg x Zn 1-x O [10,11,30].
More evidence of Mg inter-diffusion can be observed in the SIMS depth profile of the as-deposited and various temperature annealed Mg x Zn 1-x O/ZnO bi-layers, shown in Fig. 3 Whereas a significant Mg diffusing, from top Mg x Zn 1-x O into the underlying ZnO layer, is present in the 800 ºC annealed Mg x Zn 1-x O/ZnO bi-layer, shown in Fig. 3(c).The Mgdiffusion results in decreased Mg concentration from ~30% for the as-deposited (Fig.

Conclusions
The tunable detection band of MSM-PDs with Mg x Zn 1-x O/ZnO bi-layer was fabricated.By varying the annealing temperature from 700 to 900 ºC during fabrication, we can modulate the detection band of the fabricated MSM-PDs from two bands into one band.When the annealing temperature is lower than 900 ºC, two distinct detection bands were achieved due to the absorption of Mg x Zn 1-x O and the underlying ZnO layers.When the annealing temperature was raised to 900 ºC, only one detection band was observed in the NSM-PDs.This is because the Mg x Zn 1-x O/ZnO bi-layer is completely mixed into one Mg x Zn 1-x O layer owing to the diffusion of Mg atoms from Mg x Zn 1-x O to the underlying ZnO layer.PL measurement show that there are two emission peaks in the as-deposited and 700-800 ºC annealed Mg x Zn 1-x O/ZnO bi-layer.However, only one emission peak was found in the 900ºC annealed Mg x Zn 1-x O/ZnO bi-layer.Equations should be centred and should be numbered with the number on the right-hand side.

Fig. 1 .Fig. 2 .
Fig. 1.Dark I-V characteristics of the fabricated MSM-PDs with as-deposited and various-temperature-annealed Mg x Zn 1-x O/ZnO bi-layer . It is clear that the top Mg x Zn 1-x O and bottom ZnO layers are present in the as-deposited Mg x Zn 1-x O/ZnO bi-layer with Mg concentration of about 30%, shown in Fig. 3(a).The sharp interface between Mg x Zn 1-x O and ZnO leads two detection bands in the MSM-PDs with the as-deposited Mg x Zn 1-x O/ZnO bi-layer, shown in Fig. 2. No significant Mg diffusion is seen in the 700 ºC annealed Mg x Zn 1-x O/ZnO bi-layer, shown in Fig. 3(b), which causes a similar responsivity between the MSM-PDs with as-deposited and 700 ºC annealed Mg x Zn 1-x O/ZnO bi-layer, also shown in Fig. 2.

3
(a)) to ~23% for the 800 ºC annealed Mg x Zn 1-x O/ZnO bi-layer, shown in Fig. 3(c).The decrease in Mg concentration causes a red-shift of the Mg x Zn 1-x O absorption edge; the Mg atom also diffuses into the underlying ZnO layer, generating a blue-shift of the ZnO absorption edge.On further increasing the annealing temperature to 900 ºC, the Mg x Zn 1-x O/ZnO bi-layer gets homogenized into a single Mg x Zn 1-x O layer with ~8% Mg distributed almost uniformly as shown in Fig. 3(d), leading to only one detection band in the 900 ºC annealed MSM-PDs, shown in Fig. 2.

Fig. 3 .
Fig. 3. SIMS depth profile of the as-deposited and various temperature annealed Mg x Zn 1-x O/ZnO bi-layer The normalized PL spectra of the as-deposited and various-temperature-annealed Mg x Zn 1-x O/ZnO bi-layer are shown in Fig. 4. The as-deposited Mg x Zn 1-x O/ZnO bi-layer

Fig. 4 .
Fig. 4. Normalized PL spectra of the as-deposited and various-temperature-annealed Mg x Zn x-1 O / ZnO bi-layer This is because the crystalline property of the Mg x Zn 1-x O film is largely improved after annealing due to the full width at half maximum being drastically reduced from 0.54 o to 0.36 o for the as-deposited and 700 ºC annealed Mg x Zn 1-x O layers, respectively.Mg x Zn 1-x O is the top layer, and the incident light is illuminated from the Mg x Zn 1-x O side in the PL measurement.When raising the annealing temperature to 800ºC, the emission wavelength of Mg x Zn 1-x O is red-shifted and that of the ZnO is blue-shifted, due to the Mg diffusing across Mg x Zn 1-x O/ZnO interface, shown in Fig. 4(c).On further increasing the annealing temperature to 900ºC, only one PL peak occurs at about 355 nm, due to the completely homogenized Mg x Zn 1-x O layer generated, as shown in Fig. 4(d).