JP2013115114A - Pn-junction element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pn-junction element that has a p-type semiconductor layer capable of being suitably and thinly formed at a low temperature and having high hole injection efficiency.SOLUTION: A ZnO layer 2 is formed on a substrate 1, a p-type semiconductor layer 3 and an electrode layer 6 are formed on the ZnO layer 2, and a NiO layer 4 and an electrode layer 5 are sequentially stacked on the p-type semiconductor layer 3, thereby forming a pn-junction element. The p-type semiconductor layer 3 is formed from a NiMgO-based material by spattering a mixed material of NiO and MgO as a spatter target. In order to obtain diode characteristics without reducing current, the value of X in NiMgO is preferably within the range of 0.32≤x≤0.57.

Description

  The present invention relates to a pn junction element, and more particularly to a pn junction element that injects holes into a zinc oxide (ZnO) -based semiconductor light emitting material.

  A ZnO crystal is a direct transition semiconductor having a wide band gap of about 3.37 eV, and the binding energy of excitons in which holes and electrons are combined in a solid is as large as 60 meV, and it exists stably even at room temperature. It is inexpensive and has a low environmental impact, and is expected as a material for light-emitting devices in the blue to ultraviolet range. In addition to the light emitting device, the ZnO crystal has a wide range of uses, and is expected to be applied to light receiving elements, piezoelectric elements, transistors, transparent electrodes, and the like.

In order to use in these applications, it is important to establish a high-quality ZnO crystal growth technique with excellent mass productivity, and also a doping technique for controlling the conductivity of the semiconductor.
In particular, in developing a ZnO device in which a p-type ZnO-based semiconductor layer is stacked on an n-type ZnO semiconductor layer, making ZnO p-type has become a major issue. Focusing on p-type.

  For example, a method of using a group V element as a p-type doping material for doping a ZnO-based semiconductor and replacing an oxygen atom with a group V element has been studied by many organizations. N (nitrogen), As (arsenic), P ( Phosphorus), Sb (antimony), and the like are listed as candidates. Among these, N has the same ionic radius as oxygen and is a promising candidate for a p-type dopant for ZnO (Patent Document 1).

  On the other hand, a light emitting device suitable for a large screen display is also required. Therefore, there is a need for a technique for forming a light emitting element in which an n-type ZnO semiconductor film and a p-type ZnO semiconductor thin film are stacked on a substrate that is likely to have a large area, such as a glass substrate (Patent Document 2).

JP 2005-223219 A JP 2003-273400 A

  However, in order to obtain high crystallinity and surface smoothness in the p-type semiconductor film doped with nitrogen in ZnO as described above, for example, as disclosed in Patent Document 1, it is 300 ° C. to 800 ° C. Although it is necessary to perform an annealing process at a high temperature of about 0 ° C., since the glass substrate cannot withstand the high temperature, it is difficult to form a p-type ZnO-based semiconductor thin film on the glass substrate by a nitrogen doping method.

It is known that a NiO thin film is useful as a p-type material that can be formed at a low temperature.
In addition, using a mixed crystal material of ZnO and NiO has been proposed by utilizing the fact that a NiO thin film can be made p-type relatively easily at a low temperature.
However, since the NiO thin film has a large valence band offset of about 1.6 eV with respect to ZnO (N-type semiconductor), the hole injection efficiency of current injection type light emitting devices using ZnO-based materials is reduced. End up. On the other hand, in a mixed crystal thin film such as ZnNiO, the hole injection efficiency with respect to a ZnO-based material tends to decrease because the hole concentration rapidly decreases as the ZnO component with respect to NiO increases.

  In view of the above problems, an object of the present invention is to provide a pn junction element in which a P-type semiconductor layer in contact with an n-type semiconductor material can be satisfactorily formed even at a low temperature and a good hole injection efficiency can be obtained.

A pn junction element according to an aspect of the present invention has a valence band top in contact with an N-type semiconductor material and having an offset from the top of the valence band of the N-type semiconductor light-emitting material of less than 1.6 eV. Is provided with a _first p-type semiconductor layer made of a material represented by Ni _1-x Mg _x O (0 <x <1), and is disposed on the first p-type semiconductor layer (on the surface of the N-type semiconductor light emitting material). The second p-type semiconductor layer having a higher hole concentration than the first p-type semiconductor layer is disposed on the opposite side of the first p-type semiconductor layer.

According to the pn junction element of the above aspect, the first p-type semiconductor layer in contact with the N-type semiconductor material has a valence band top whose offset from the top of the valence band of the N-type semiconductor material is less than 1.6 eV. Therefore, the hole injection efficiency is good.
Further, since the first p-type semiconductor layer is made of a material whose composition is represented by Ni _1-x Mg _x O (0 <x <1), a thin layer can be satisfactorily formed even at a low temperature.

For ZnO and _{Ni 1-x Mg x O (} 0 ≦ x ≦ 1), is a graph showing the results of measuring the electron state in the valence band as viewed from the vacuum level in XPS. For ZnO and _{Ni 1-x Mg x O (} 0 ≦ x ≦ 1), a diagram showing the energy position of the valence band top and the energy position of the conductor bottom when viewed from the vacuum level. It is a figure which shows an example of the pn heterojunction element using MgNiO type material. It is a figure which shows the result of having measured the voltage-current characteristic about the pn junction element concerning an Example.

A pn junction element according to one embodiment of the present invention has a valence band top that is in contact with an n-type semiconductor material and has an offset amount of less than 1.6 eV from the top of the valence band of the n-type semiconductor material. Includes a _first p-type semiconductor layer made of a material represented by Ni _1-x Mg _x O (0 <x <1), and is formed on the first p-type semiconductor layer, that is, the first p-type semiconductor layer. A second p-type semiconductor layer having a hole concentration higher than that of the first p-type semiconductor layer is disposed on the side opposite to the side facing the N-type semiconductor light emitting material.

  According to this pn junction element, the first p-type semiconductor layer in contact with the n-type semiconductor material has a valence band top whose offset from the valence band top of the n-type semiconductor material is less than 1.6 eV. The hole injection efficiency is good. In addition, since the first p-type semiconductor layer is made of a material whose composition is represented by Ni1-xMgxO (0 <x <1), a thin layer can be formed well even at a low temperature.

The n-type semiconductor material is preferably made of zinc oxide (ZnO), and the second p-type semiconductor layer is preferably made of NiO.
In the Ni _1-x Mg _x O constituting the first p-type semiconductor layer, x is preferably 0.32 or more and 0.57 or less.
Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments.

<Embodiment>
(P-type semiconductor material)
First, a p-type semiconductor material composed of Ni, Mg, and O according to the present invention will be described.
As a result of detailed studies, the present inventor has shown that the p-type semiconductor material represented by Ni _1-x Mg _x O has excellent film-forming properties at a low temperature. Therefore, even at a relatively low temperature of 500 ° C. or lower, It has been found that a low-resistance thin film can be formed on an n-type semiconductor layer, and therefore this p-type semiconductor material can form a p-type semiconductor layer on a substrate having low heat resistance such as a glass substrate. .

Further, by stacking the p-type semiconductor material on the ZnO layer, an element in which the p-type semiconductor material layer and the n-type ZnO layer are heterojunction can be formed. It has also been found that a light emitting element can be formed.
Here, in order to form the p-type semiconductor material in the form of a thin film, a mixed material of NiO and MgO may be sputtered on a substrate or a ZnO layer.

Note that since this thin film formation is likely to be n-type when performed in a reducing atmosphere, it is preferable to form a p-type semiconductor film in an oxidizing atmosphere.
The material having the above composition has a p-type semiconductor property because an element such as Ni having 3d electrons in the outermost shell and having an energy level of 3d orbital higher than that of 4s orbital is mixed with MgO. This is probably because holes are easily formed in the 4s orbit.

The p-type semiconductor material preferably has a composition represented by Ni _1-x Mg _x O (0 <x <1). Here, Ni _1-x Mg _x O is an oxide in which NiO and MgO are mixed, and x is the ratio of the number of moles of Mg to the total number of moles of Ni and Mg.
The p-type semiconductor material may be in an amorphous state, but is preferably a crystalline compound in order to obtain excellent characteristics.

When the p-type semiconductor material is a crystalline compound, it may be a mixed crystal in which Zn in the ZnO crystal is partially replaced with Mg, or a mixed crystal in which Mg in the MgO crystal is partially replaced with Zn, or a ZnO crystal and MgO It may be a crystal mixture in which crystals are mixed.
The Ni _1-x Mg _x O-based material is a p-type semiconductor capable of forming a thin film at a low temperature of 500 ° C. or lower, and can form an excellent heterojunction on the ZnO layer.

FIG. 1 is a diagram showing a state in the vicinity of the valence band of ZnO, NiO, Ni _1-x Mg _x O, and MgO thin film materials obtained by XPS measurement, and will be described in detail in Examples.
In each spectrum, the energy on the horizontal axis is calibrated with the binding energy of Au4f that can be measured with a thin film in which Au is further thinly deposited by about 10 nm.
From the rise position of the spectrum observed in the region of 9 eV or less, the relative relationship of the energy position of the valence band top can be obtained. Band diagram obtained based on physical properties of ZnO, NiO and MgO, which are pure materials obtained from the past, and measured optical band gap of Ni _1-x Mg _x O thin film obtained here. Is FIG.

As is apparent from FIG. 2, the larger the value of x representing the Mg ratio, the smaller the offset amount of ZnO from the valence band top. As the offset amount is smaller, the hole injection efficiency and the reverse bias withstand voltage are increased when the ZnO layer and the pn junction element are formed. Therefore, it is preferable that the value of x is large, and to suppress it to 1.6 eV or less. 0.32 ≦ x is preferable. On the other hand, the value of X in Ni _1-x Mg _x O is preferably 0.57 or less in order to make the electric conductivity type of Ni _1-x Mg _x O p-type and keep electric resistance low.

That is, a thin film material with a Ni _1-x Mg _x O X value of 0.57 or more is preferred as a low-current device application with priority given to the withstand voltage performance against reverse bias considering application to an optical sensor or the like. In the case where priority is given to hole injection efficiency even at the expense of a small amount of reverse bias characteristics, such as LED lighting, a thin film material having a value of X in Ni _1-x Mg _x O of 0.57 or less is suitable. is there.

In addition, when applying Ni _1-x Mg _x O to displays, etc., it is necessary to keep both hole injection efficiency and withstand voltage performance against reverse bias good. In addition, an optimal value of X may be selected.
(Configuration of pn junction element)
FIG. 3 is a diagram illustrating an example of the pn junction element according to the embodiment.

  As shown in this figure, this pn junction element has a ZnO layer 2 formed on a substrate 1, a p-type semiconductor layer 3 and an electrode layer 6 formed on the ZnO layer 2, and a p-type semiconductor layer. 3, a NiO layer 4 and an electrode layer 5 are sequentially stacked on the p-type semiconductor layer 3 (the side opposite to the side facing the ZnO layer 2 in the p-type semiconductor layer 3). The p-type semiconductor layer 3 is made of a Ni1-xMgxO-based material.

In this pn heterojunction element, a p-type semiconductor layer 3 made of Ni1-xMgxO is interposed as an intermediate layer between the ZnO layer 2 and the NiO layer 4.
The effect of the intermediate layer made of Ni _1-x Mg _x O will be described.
Although the NiO layer 4 has a sufficient hole concentration, since the band offset with respect to the valence band of the ZnO layer 2 is large as shown in FIG. 2, the hole injection effect cannot be obtained if the NiO layer is directly joined to the ZnO layer.

In contrast, Ni _1-x Mg _x O has an energy level at the top of the valence band between the energy level at the top of the valence band of ZnO and the energy level of the top of the valence band of NiO. Since the p-type semiconductor layer 3 is interposed between the ZnO layer 2 and the NiO layer 4 as described above, the amount of hole injection can be increased, and an efficient hole injection structure can be realized.

That is, in the pn junction element, the high potential barrier against carriers (holes) between the ZnO layer 2 and the NiO layer 4 is separated into a plurality of steps to improve the carrier conductance. Can do.
In the pn junction element, there is only one p-type semiconductor layer 3 that is an intermediate layer between the ZnO layer 2 and the NiO layer 4. In this case, the valence band energy that can equally divide both potential barriers. It is desirable to insert material with a position.

The pn junction element can be used as a light emitting element that emits light having a wavelength from a blue region to an ultraviolet region.
Further, since the p-type semiconductor layer 3 can be formed even at a low temperature of 500 ° C. or lower, a glass substrate can be used as the substrate 1. Therefore, the pn junction element is suitable for forming a large screen display.

As an example, a mixed material of ZnO and NiO was produced as a sputtering target, and a Ni _1-x Mg _x O thin film was produced by sputtering on a substrate.
NiO and MgO used as raw materials have a purity of about 99.99%. When mixing NiO and MgO, the molar ratio of MgO to the total number of moles of NiO and MgO was 0.25, 0.5, and 0.75.

The substrate temperature was 200 ° C., the introduced gas was an Ar: O ₂ flow ratio of 1: 1, the pressure was 1 Pa, and the input power was 600 W. The thickness of the thin film was about 200 nm.
As a comparative example, NiO alone and MgO alone were similarly formed as thin films.
About each thin film concerning an Example and a comparative example, the band gap measurement by XPS measurement, the determination of p-type n-type by the thermoelectromotive force method, and the light transmittance measurement was performed as follows.

Analysis by XPS:
As a result of analyzing the XPS spectrum as a measurement result by XPS, the ratio of the number of moles of Mg to the number of moles of Ni + Mg was 32%, 57%, and 78%. This thin film composition almost reflected the composition of the sputter target.
For each thin film, the electronic state of the valence band when measured from the vacuum level was measured.

The result is shown in FIG. From this result, it can be seen that as the amount of MgO added to NiO increases, the position of the valence band top from the vacuum level becomes deeper and approaches MgO.
Determination of p-type and n-type by the thermoelectromotive force method:
About each thin film, the p-type n-type was measured with the thermoelectromotive force method.
As a result, the thin films according to the examples all showed p-type. Moreover, although the NiO thin film of the comparative example also showed a p-type, it could not be determined for the MgO thin film.

Band gap measurement:
The transmittance spectrum of the Ni1-xMgxO (X = 0.32, 0.57, 0.78) thin film and the NiO thin film was measured with a spectrophotometer, and the band gap (Eg) of each thin film was obtained. The values are shown in Table 1. In addition, literature values are described for MgO.

また、図１と表１の結果から、各薄膜について、価電子帯トップと伝導体ボトムの位置を求めた。その結果を図２に示す。なお、図２の中でＺｎＯに関しては、文献値に基づいて記載している。

From the results of FIG. 1 and Table 1, the positions of the valence band top and the conductor bottom were determined for each thin film. The result is shown in FIG. In FIG. 2, ZnO is described based on literature values.

From the results shown in FIG. 2, the position of the valence band top in Ni _1-x Mg _x O gradually approaches the position of the valence band top of ZnO as x increases. That is, it can be seen that the offset amount from the top of the valence band of ZnO is small.
Voltage-current characteristics of the element:
Using a Ni _1-x Mg _x O-based material (x = 0.32, 0.57, 0.78) for the p-type semiconductor layer 3, a pn junction element having the structure shown in FIG. , X = 0.32, Example 2 has X = 0.57, and Example 3 has x = 0.78 As a comparative example, x = 1 (NiO) was also produced.

The voltage-current characteristic was measured about each produced element.
FIG. 4 is a characteristic diagram showing the result, showing the relationship between the applied voltage and the current.
In FIG. 4, in the comparative example, it can be seen that the graph has a symmetrical shape with respect to the voltage 0 and the rectification characteristic as a diode is not obtained.
On the other hand, in Examples 1 to 3, when the voltage is negative, the current is small and positive when the voltage is positive, and diode characteristics are obtained.

This result shows that the diode characteristics can be obtained by interposing the Ni _1-x Mg _x O layer (Examples 1 to 3) between the ZnO layer and the NiO layer.
In FIG. 4, the current value in Example 3 is lower than that in Examples 1 and 2, and the current value is lower than that in the comparative example. From this, it can be seen that the value of x is preferably 0.57 or less in obtaining the current value.

From the above, it can be said that the value of X in Ni1-xMgxO is preferably in the range of 0.32≤x≤0.57 in order to obtain diode characteristics without reducing the current.
However, in consideration of the trade-off relationship between the hole injection efficiency and the diode characteristics, an optimal value of X may be selected in accordance with the device specifications.
For example, when importance is attached to the diode characteristics, a range of 0.57 ≦ x ≦ 0.78 may be set.

(Modification)
In the above embodiment, the ZnO layer 2 is formed on the substrate 1, and the p-type semiconductor layer 3 and the NiO layer 4 are sequentially stacked on the ZnO layer 2 to form a pn junction element. A material in which the p-type semiconductor layer 3 and the NiO layer 4 are sequentially laminated on the surface of the particulate ZnO material can be formed on a substrate to form a pn junction element, which is used as a light emitting element. You can also.

  The pn junction element according to the present invention can be used in a wide range of fields such as a display and illumination as a light emitting device from a blue region to an ultraviolet region.

1 substrate 2 ZnO layer 3 p-type semiconductor layer 4 NiO layer 5 electrode layer 6 electrode layer

Claims (3)

In contact with the n-type semiconductor material, the N-type semiconductor material has a valence band top whose offset from the top of the valence band is less than 1.6 eV, and the composition is Ni _1-x Mg _x O (where 0 A first p-type semiconductor layer made of a material represented by <x <1) is disposed;
A pn junction element in which a second p-type semiconductor layer having a higher hole concentration than the first p-type semiconductor layer is disposed on the first p-type semiconductor layer. The n-type semiconductor material is made of zinc oxide,
The pn junction element according to claim 1, wherein the second p-type semiconductor layer is a hole injection layer made of NiO. 3. The pn junction element according to claim 1, wherein x is 0.32 or more and 0.57 or less in Ni _1-x Mg _x O constituting the first p-type semiconductor layer.

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