Magnetic bipolar transistor based on ZnO/NiO/Si heterostructure using pulsed laser deposition

For next generation spintronic devices, antiferromagnetic materials are potential candidates due to their privileged properties of zero net magnetization which leads to no stray fields and cannot be affected by external magnetic field. They manifest clearly spin-orbit and magneto-transport effects due to intrinsic high frequency dynamics. Recently, it opens a roadmap to analyse spin-based technologies using these novel concepts in antiferromagnets. Transition metal oxides (TMOs) have enticed much heed in research fields due to their diverse potential technological applications.^1–3 TMOs like ZnO and NiO are further beneficial due to their unique properties of transparency and conductivity which make them suitable for various applications in optoelectronics, LED, solar cell, gas sensors, and thin film transistors.^4–10 NiO which is an antiferromagnetic material is a promising p-type TMO with wide band gap of 3.6-4 eV and demonstrating tunable electrical properties. Zinc oxide which is an n-type wide band gap (E_g = 3.3 eV) semiconductor have shown potential applications in numerous device applications like optoelectronic devices, photovoltaic, spintronics, and sensors.^11,12

Bipolar junction transistor (BJT) has the capability to control the flow of current between the three terminals of transistors which allows them to serve as the building block for various logic devices, amplifiers and various electronic devices such as mobile phones, television, and radio transmitters.^13,14 BJTs have high driving capability and can operate at high frequencies enabling their applications in transparent displays, functional windows, or augmented-reality devices. In the present work, aNPN bipolar transistor have been fabricated by depositing thin films of a p-type NiO and an n-type ZnO on n-type Si wafer using pulse laser deposition technique (PLD). PLD has been proved to be a highly efficient and flexible technique for depositing various metals and oxides such as ZnO and NiO. The structural, optical and magnetic properties of the thin films were done and analysed. The electrical properties of the fabricated device were studied in diode and transistor mode. Various parameters such as ideality factor (n), series resistance (R_S), and q-point were determined using standard methods.

The thin films of n-ZnO/p-NiO were grown by pulse laser deposition (PLD) technique on 5×5 mm² n-type silicon wafer. NiO and ZnO targets were used for the formation of NiO and ZnO layers and at the time of deposition the energy and pulse rate of laser was kept at 200 mJ and 10 Hz, respectively. The films were deposited for 10 minutes with an elevated temperature of 670°C. During the deposition, the energy density of the laser beam was 1.68 J/cm². It was followed by six hours annealing at the same temperature of 670°C. The growth and post annealing temperature plays a crucial role for obtaining high quality thin films as the surface roughness decreases significantly at higher temperatures.^15,16 The distance of the substrate from the target was 4cm, laser shots of around 4000 were given for both NiO and ZnO. The base pressure and oxygen pressure was kept at 6x10⁻⁶ mbar and 0.1 mbar, respectively. The schematic of the fabricated device is shown in Fig. 1(A). Thickness of NiO base region was kept much thinner than the ZnO collector region.

Rigaku Miniflex X-ray diffractometer (XRD) was used to study the structural properties of deposited thin films using the 2θ - θ scan with Cu-k_α. The Raman spectra were recorded using Enspectr Raman spectrometer in the range 300-1200 cm⁻¹ using a green laser source with a power of 300 mW and a wavelength of 532 nm. The magnetic measurements were observed by using vibrating-sample magnetometer (VSM) Cryogenic Ltd UK. I-V output characteristics were measured for both diode and transistor using a KeysightB2902A precision source/measure unit (SMU).

The phase formation of deposited NiO and ZnO films has been determined by XRD. Fig. 1(B) elucidates the XRD spectra of NiO and ZnO thin films deposited on an n-type Si substrate. As it is cleared from the results, ZnO shows preferred orientation along the (002) direction and is highly crystalline. On the other hand, the NiO films show polycrystalline behaviour with preferred orientation along the (111) and (200) direction.¹⁷ 

The Raman spectra were obtained using the Raman spectrophotometer, in which the excitation laser of wavelength 532 nm with linearly polarized beam was focused on the sample. The parallel component of polarized laser was focussed on the substrate as the perpendicular part of the polarized laser gets cancel out. Since NiO is an antiferromagnetic material, it has spin up and spin down type of magnons associated with it which will depend upon the structure of NiO at ground state. Fig. 1(C) shows that the Raman peak of NiO at 530 cm⁻¹ corresponds to first order longitudinal optical (1P) and at 1090cm⁻¹ to the second order longitudinal optical (2P). This is due to Ni-O stretching mode and indicating a strong phonon-magnon interaction at 530 cm⁻¹. Similarly, for ZnO, the Raman peak at 430 cm⁻¹ corresponds to first order longitudinal optical and the second order longitudinal optical at 1100 cm⁻¹.^18–20 The intensity of 2P peak is nearly close to 1P peak which reveals the presence of some imperfections in the crystal quality of ZnO thin film. Hence, Raman spectroscopy provides further information about the crystal structure quality and lattice defects present in the device structure.

Fig. 1(D) reveals the magnetic hysteresis loop of ZnO/NiO/Si heterostructure with external magnetic field applied parallel to the film plane. It is clear from the loop that ferromagnetism was present in the fabricated device which may be due to the presence of nickel or oxygen vacancies in NiO contributing to spin polarization.^21,22 The in-plane coercivity was found to be 42 Oe.

We have theoretically studied the effect of Ni or O vacancy in NiO by using Density Functional Theory (DFT) (Fig. 2). It reveals that the pristine NiO is antiferromagnetic in nature, whereas, the vacancy induces ferromagnetism in NiO which agrees well with the previous report.²³ However, the magnetic moment (0.018μ_B) is much higher with a very small Ni vacancy (∼3%). Whereas, the moment generated in for ∼ 13% O vacancy is about 0.0022μ_B, which is very small in comparison to Ni vacancy.

The electrical characterization of NiO and ZnO films grown on Si wafer was carried out at room temperature. Fig. 3 (A and B) shows the I–V characteristics of the two fabricated diodes i.e. p-NiO/n-Si and n-ZnO/p-NiO with the forward and reverse biasing. It is evident that the fabricated device shows the diode characteristics which can be described by physics of semiconductors. The current in which is given by following equation:

where q is the electronic charge, V is the applied voltage, k is the Boltzmann constant, T is the temperature in Kelvin, n is the ideality factor and I_S is the reverse saturation current flowing through the depletion region irrespective of the type of polarization. Generally, the current equation is modelled by following equation:²⁴ 

where R_S is the series resistance whose higher value depicts deviation from the ideal diode behaviour. V_T is kT/q is the thermal voltage. The ideality factor for NiO/Si have been obtained to be n = 4.56 using Eq. (1) and series resistance was obtained to be R_s=0.066 MΩ from Eq. (2). Similarly, for NiO/ZnO junction the calculated values are n = 3.25, and $Rs=0.11MΩ$⁠. Fig. 3 (C–D) shows the I-V characteristics of the fabricated diodes for various magnetic fields applied perpendicular to the film plane. It is clear from the results that the knee voltage decreases with the increase of magnetic field. This is an indication of induced ferromagnetism from Ni or oxygen vacancies in NiO thin film. In forward bias region, at a particular applied voltage, current increases with magnetic field.

We have explained the above behaviour by using a spin splitting mechanism as shown in Fig. 3(E and F). Fig. 3(E) shows a schematic of the spin splitting under a magnetic field. It is evident from Fig. 3(F) that, at $B→=0$ when there is no spin splitting, the built-in potential $(Vbi)$ for both spin-up and spin-down energy levels are relatively high which basically produces a low forward bias current. However, when the spin splitting increases under a magnetic field (⁠$B→≠0$⁠), $Vbi$ for both the spin no longer remain same as before. For example, $Vbi+1/2$ significantly decreases and $Vbi−1/2$ increases with respect to $Vbi$ as can be seen in Fig. 3(F). This, indeed decreases the knee voltage of the pn junction and significantly increases both the spin-up electron and hole transport in the forward bias condition.²⁵ Therefore, the overall current increases under a magnetic field. It can also be understood from the energy band diagram that the nature of the forward bias current will remain same if we just reverse the magnetic field (e.g. H = −1400 Oe). Thus, we can conclude that the device under study can be used as a magnetically tunable semiconductor device.

In the proposed magnetic transistor, we used the forward active regime of the transistor where the emitter-base depletion layer is forward biased and base-collector junction is reverse biased. Fig. 4(A) reveals the output characteristics of the fabricated transistor in common emitter configuration for various applied bias current (I_B). For low base current, initially with the increase in output voltage (V_ce) collector current (I_c) increases linearly since for low voltages the junction is not reverse biased. When the supply increases the base-collector junction becomes reverse biased and I_c increases very little with V_ce and becomes saturate with the output voltage.²⁶ Thus the fabricated device shows the transistor characteristics and the q-point was found to be in the active region at a voltage of 2.17 V and a collector current of 1.73 μA. Fig. 4(B) shows the output transistor characteristics as a function of magnetic field. It is observed that with the increase of magnetic field, both in + and – directions, the collector current increases which can be attributed due to the presence of ferromagnetism from Ni or oxygen vacancies in NiO thin film in the presented transistor as explained above in case of the diode characteristics. Therefore, we can conclude that such devices can be utilized for future spintronics applications.

We designed magnetic bipolar transistor ZnO/NiO/Si heterostructure using pulsed laser deposition technique. The structural analysis revealed the phase formation of NiO and ZnO thin films. Raman spectroscopy demonstrated the 1P and 2P peaks for both NiO and ZnO which indicate better crystal quality of the films. The ferromagnetism observed in the magnetic hysteresis loop can be attributed due to the presence of either Ni or oxygen vacancies in NiO thin film. I-V characteristics of the fabricated diodes showed non-linear behaviour. The fabricated diode showed decrease in knee voltage with the applied external magnetic field. The transistor characteristics were determined in CE configuration. The fabricated transistor showed increase in collector current with the increase of magnetic field which was explained in terms of spin splitting under an applied magnetic field.

This work is supported by the MHRD-IMPRINT grant, DST-SERB, DST-AMT grant of Govt. of India.